It's weirder still because you often don't actually have to register with a party, even to vote in the primaries like other commenters have mentioned; it depends on which state and which party. [0]

For example, I just voted in the Minnesota primaries. Ours are open primaries, so you don't need to register with any party and the ballot has every party on it. Though, it spoils your ballot to vote in multiple parties' primaries. So you gotta choose, but you do that privately.

[0] https://ballotpedia.org/Primary_election_types_by_state

It's because we have open primaries in the US - registering as a Democrat or Republican is only important as far as voting in primaries. In other countries the selection of candidates is internal to the political parties themselves. There are pros and cons for both systems but the US system is fairly unique globally aside from a few exceptions in South America.

It seems quite weird. Our parties are generally quite weird because were are not supposed to exist in the first place and so most of the rules are sort of ad-hoc.

I don’t belong to one, but it is sort of annoying to not be able to vote in the “primary elections” (where the parties pick their candidates). My state has a strong partisan lean so this is the only election that actually matters here. Oh well.

Because by and large they're all trapped in an "us vs them" team sport where both sides believe everybody on the opposing team is literally Satan.

Then again, EU citizens would find it questionable how in the UK recruiters are asking about your ethnicity, disability status etc.