This is what you call an “opinion.” Since it’s coming from the soon to be POTUS, it obviously holds a bit more weight, by (sic), it’s an opinion. I personally disagree with it, holding the opinion that you should be allowed to burn the flag, and, if so, someone should be allowed to punch you in the face once without facing assault charges.
Source: NY Times Is Rather Vexed By Trump’s Tweet On Flag Burning | John Hawkins’ Right Wing News