That assumes a dichotomy of either: everything is allowed, or Twitter blocks things they want to block.

There are other forms of moderation that might be acceptable to people who are otherwise free speech absolutists. Crowd-sourced block lists (i.e. subscribe to a list of accounts marked by other users as spam), with the option to introduce your own exclusions to the list.

Alternatively, a web-of-trust model where you only see tweets & replies from people you follow. Or maybe the people you follow + the people they follow. Or maybe configure it on a per-person basis, if you trust person A's followee list more than person B's.

There are a ton of options. The point is that users don't currently have a choice, and have to deal with Twitter's policies with no opt-out or whitelisting.