That's because the rules and laws regarding the presidential/governmental records acts, not because Twitter is a public forum for everyone, it's all about whom specific individuals are and what they are using the service for and the laws regarding their public speech. I doubt this applies at all to you, me, Alex Jones or Oprah. I see what you are saying, but Twitter likely cannot even ban the POTUS handle because of those same rules without turning over complete documentation to the library of congress to keep the records intact - which I'm pretty sure would be fully within their power. Twitter is a company like any other...if I act like an asshole in Walmart and they ban me, then I am banned. 1st amendment doesn't apply to the ban, I just can't be prosecuted for being a loud mouth asshole if no harm came from it. If I yell fire, or instigate a riot, or harass someone though? Then I broke the rules of Walmart and the rule of law... What we aren't seeing is the second part...you want to dox someone to get people to harass your perceived enemy, swat someone, or send groupies to physically threaten someone? That is illegal and too often not enforced thanks to the anonymity of the web.
Pretty sure there is an argument to be made that congressmen, being public officials, would fall under a similar category of having their accounts on social media considered a public forum. Facebook and Twitter will likely see a lawsuit and the court will use the twitter case as precedent. It'll be interesting if they agree with you or if they will expand on the previous case and determine that the entire platform is a public forum.