Because a lot of power isn't projected through laws. If you take Hitler's Enabling act, parliamentarians that didn't vote for it, did get punished but there was no need to explicitely write that into the law.
A lot of these examples are distinct from Roko's idea, in that they are self-reinforcing, but generally through other mechanisms than distinguishing supporters from non-supporters and targeting those groups specifically.
There's a pretty strong governance norm (and in many cases constitutional protection) against this kind of segregation and targeting, at least in nominally-free democratic societies. A politician who puts opponents in jail JUST because they are opposed (or proposes a law that punishes ONLY those who oppose it) won't last long in most civil societies. In fact, the ability to do stuff like this is a pretty strong indicator that civility is a sham in that area.
This, of course, doesn't apply to a hypothetical all-powerful AI, as it doesn't really care about democratic support or what its' subjects think.
Surely it was, but in slightly different form, in which it is rather trivial: When a person says "If I win the election I will give everybody X".
But that's not Roko's basilisk. Whether or not you individually vote for the candidate does not affect you as long as the candidate wins.
I think something like this actually happens a lot in politics. To give a contemporary example, in many places in the West currently, the best way to protect oneself against being politically attacked in the future via accusations of various "isms" and "phobias" is to prove to be an "ally" by loudly accusing others of "isms" and "phobias" or otherwise helping the faction that tends to make such accusations to gain more power.
(Posting as a comment because it's not exactly what you're looking for, i.e., a bill or law.)
Why has there never been a "political Roko's basilisk", i.e. a bill or law that promises to punish any member of parliament who voted against it (or more generally any individual with government power, e.g. judge or bureaucrat, who did not do everything in their capacity to make it law)?
Even if unconstitutionality is an issue, it seems like the "more general" condition would prevent judges from overturning it, etc. And surely there are countries with all-powerful parliaments.