It has always been both legal and appropriate to investigate the relationship between client and lawyer if there is sufficient reason to suspect that illegal behavior has been colluded upon by both. And this is exactly what is being suspected in this instance.
For example, if a lawyer and his client commiserate on a plan to bribe a juror or a judge to win a trial, their collusion is not covered under the attorney-client privilege provision. And if law enforcement suspects that such has happened, then they are allowed to search for and seize evidence to prove it.
It is suspected that Trump and his lawyer conspired to use campaign money to pay off hookers so they would't sabotage their political campaign by telling their stories in public. This is clearly illegal. And as such law enforcement has the right to search for and seize evidence to prove or disprove this illegal behavior. (Only elected politicians get to use public money to buy their whore's and victim's silence.)
The Trumpsters don't bother to learn the facts, however. They are notorious and willfully ignorant of what's really happening, and why, so they can maintain their absurd and bigoted fantasies about state overreach.