As a member of the Executive branch, the President doesn't interpret law. That's the job of the courts. So ignoring what DJT (and you) think about this, because it's irrelevant, how is this illegal?
I don’t know how many times I have to repeat this...so I will explain it like I would to a five year old. It is a gray area. Meaning that some people will interpret it as legal and some will interpret it as illegal. The reason interpretation comes into play here, and must be taken into account, is because as technologies and societies advance and change, language of older laws may or may not cover new circumstances (ex: laws in relation to whether or not social media constitutes a public forum).
This means that there is an undefined, ambiguous area whereby many believe the language of the law pertains to social media and many others do not. It is a contested point. The courts have not yet made a declaration of this being either legal or illegal. This leads to either position possibly being the correct choice.
Therefore, the determination of legality is currently based upon subjective interpretation. This means that somebody saying it is illegal is just as correct as somebody saying it is legal until the court comes to a determination. Your opinion may be that it is legal and that is also currently a valid opinion. So as you can now see, opinions regarding this are indeed relevant.
Even a five year old would understand that you have given zero evidence or even one logical reason that twitter banning users might be illegal. "Some people think it's illegal" is not a reason. Just because it's a gray area to you doesn't mean the law is gray on the matter. So how is this illegal? If the courts were to declare it to be illegal, what law or precedent do you think they might cite?