The First Amendment allows Trump to express his views without any legal penalty for it — unless this expression is slanderous, libelous, or creates some kind of imminent danger (such as shouting “fire!” in a crowded theater).
The First Amendment also allows those of us reading the expression of Trump’s views to comment on them by pointing out that they are childish, bullying, and completely un-Presidential.
There’s an art to criticism. For example, the review of Trump Grill was critical, but it pointed out specific things, and why they were worthy of criticism.
Trump’s tweet, on the other hand, was long on vague and random name-calling and short on specifics.
Trump is entitled to tweet. We are entitled to say what we think about those tweets. See how it works?