For political texts in the US, it is illegal for iOS or Android to proactively treat them as spam. They're a form of political speech protected by the First Amendment.

But in my experience, they do actually slow down if you reply STOP on all of them.

It’s not illegal for them to treat them as spam proactively. In fact, manufacturers and carriers and text messaging platforms are completely free to make the decision to block them (and some do).

It isn’t a first amendment issue either actually - it’s just that legislators lobbied for an explicit exemption in the laws passed around this (after all they wrote the text). CAN SPAM is an example.