While Trump has kept his promise to crack down on illegal immigrants, this increased enforcement has apparently made life easier for criminals and more difficult for police. This is because illegal immigrants are now far less likely to report crimes to the police or assist in police investigations.
While Trump and others have claimed that immigrants come here to commit crimes (and steal jobs), the evidence shows that native citizens commit crimes at a higher rate than immigrants. Immigrants are more likely to be victims of crimes than perpetrators, which is one reason why some police departments are reluctant to serve as agents of federal immigration policy. After all, they need the cooperation of victims and witnesses to investigate crimes. This is not to say that illegal immigrants do not commit crimes; they do and this is a matter of legitimate concern. While the legal issues of immigration are obviously a matter of law, there are important moral issues here as well.
As noted above, one compelling reason for the local police and officials to not work as enforcers for federal immigration policy is that illegal immigrants who are victims or witnesses of crimes will be far less likely to cooperate with the police. From a utilitarian standpoint, this would morally problematic because it would result in more harm than good by allowing criminals to remain at large. As an example, illegal immigrants have been picked up at courthouses after serving as witnesses for the prosecution. This practice will certainly deter illegal immigrants from coming forward as witnesses. As such, this would seem to provide a moral justification for local governments to ignore the immigration status of people who have otherwise not broken any laws.
The easy and obvious counter to this line of reasoning is to point out that illegal immigrants are, by definition, all criminals. As such, ignoring their immigration status would allow criminals to remain in the community engaging in criminal activities. To add in a utilitarian element, it can be argued that while tolerating illegals who do not engage in other crimes would be a small thing, the damage to the rule of law would be significant in its harms.
One reply to this is to point out that lesser criminals are often given immunity to encourage them to testify against more important criminals. This same sort of justification could be applied here: the extremely minor crime of being an illegal immigrant can be justly ignored to ensure that the illegal immigrants are able to report serious crimes and serve as witnesses in prosecutions of such crimes. The obvious problem with this reply is that it justifies ongoing criminal activity. To use an analogy, it would be like allowing people to continuously violate minor traffic laws in the hopes that they would be more amenable to cooperating with the police regarding more significant crimes. The absurdity of this would seem to show that allowing one crime in the hopes of getting more cooperation combating other crimes is not a reasonable idea.
Another reply is that the illegal immigrants are only criminals because of bad immigration law and a defective immigration system. The gist of this approach is to argue that the immigrants who do not commit other crimes should not be classified as criminals in the first place and that enforcing such bad laws is morally wrong. It could be argued that there is a crude integrity in mindlessly obeying the law, but history has shown that “just following orders” is not an adequate moral defense. The challenge here is, of course, working out whether the immigration laws are bad laws. On the face of it, there does seem to be considerable agreement that they are not very good laws. However, it is still reasonable to consider whether the laws are bad enough to warrant regarding them as unjust laws. My own view is that the laws are bad laws but that by leaving them on the books and not enforcing them, we encourage a disrespect for the law. As such, I favor changing the laws so that they are just laws that are right to enforce.
One way to look at the matter is to consider the history of the United States: European immigrants simply showed up on the shores and started expanding into already inhabited lands. Almost any argument advanced in defense of European immigration into the New World could be dusted off and refurbished into arguments justifying the new illegal immigrants. Of course, the new illegal immigrants have a stronger moral case: they mostly coming here just to work rather than to kill the current inhabitants and take their land.
There is also the practical argument regarding law enforcement. As others have noted, the police have limited resources and it makes more sense to use those on serious crimes rather than on people who are merely here illegally and otherwise law-abiding. The moral aspect of this argument is that focusing on the more serious crimes will create more benefits than using resources to go after illegal immigrants.
My own view is that the current laws and practices regarding illegal immigration are morally unacceptable. The obvious solution involves changing the laws to match the ethics and reality of the situation and for politicians to stop making excuses and, worse, to stop exploiting the matter for short term political advantages at the expense of both the illegals and the local communities.