Twitter has the right to ban anyone for violating their ToS, including the (thankfully, ex) president of the United States. I don’t see any problem with that. You don’t have a _right_ to a space to promote your ideas from a private company, this is not how free speech works.
And no, the right to ban someone does not translate to an obligation.
What's regrettable isn't that twitter bans an ex-president, what's regrettable is that we've allowed corporations to capture the public square on the internet to the extent that they can de-facto ban the ex-president from discussion.
Where does twitter get all of its money from? Not directly from users, but instead from ads. They are not directly beholden to the interests of their customers but instead other corporations.
He was actually the sitting president at the time (during the "lame duck" period after he lost the election) and I think that matters for something. You might actually be saying this, but I wanted to inform anyone else reading.
And no, the right to ban someone does not translate to an obligation.