I don't follow your logic. If Twitter has every right to block whomever they want, what could be wrong with Apple doing the same. Does the degree of difficulty in finding a replacement change their right? Obviously, Apple isn't an opinion platform but it didn't sound like you were complaining that Apple blocked an app.
Since Apple and Google control all the phones in the US, when they block an app, they have effectively banned it from even competing. Its like Verizon AT&T and Sprint banding together and blocking all conservatives from using their cell phone networks.
What are we supposed to do? Build our own cell phone networks? Create our own phones?
Twitter can theoretically get away with blocking conservatives by claiming we can create our own forum so they're not a monopoly. But I don't think Apple and Google will be able to get away with it. They have a monopoly on the market and they blocked the voice/outlet of probably a third of the country!