"<Person> shouldn't have their home searched for purely political reasons" is a completely valid argument. But when you realize the person is the unstable former US president, he was presumably searched out of genuine suspicion (I wouldn't consider it just "politics"), and then it contained a list of foreign spies and other documents it wasn't supposed to - now it's no longer reasonable. But I would also say you're no longer really arguing "<Person> shouldn't have their home searched for purely political reasons".
The two are barely connected. Yeah they are connected, but no rational person would jump from "home search for political reasons is unjustified" to "leaking spies to enemy states is justified". An extremist would understand, but extremists can figure out codes to communicate with each other as long as any speech is allowed.
Radicalizing sensible people is absolutely a real phenomenon which is happening right now in the US, and has happened many times before. But it's not free speech which has caused this. It's actually poverty and other societal flaws which make people upset, and when the "good guys" don't fix those flaws, people start listening to the bad guys. Sensible people can listen to garbage, but they can also listen to your side, and if they're still reasonable they can make rational judgement.
It doesn't work. Whenever someone wants to say an opinion in the black area, they'll say the closest thing on the boundary of the grey area, instead. Their followers, who do the same thing themselves, understand this as if it was the original opinion in the black area (or something close to it).
A current topical example: "<Person> should be allowed to sell lists of spies to enemy states" is in the black area, but "<Person> shouldn't have their home searched for purely political reasons" is in the grey area.