I think the difference here is that the terms "top/bottom" or "dom/sub" would only be used in the context of a very specific kink-based relationship if the characters were heterosexual. While meanwhile in some BL, it seems to be standard not only to strongly flag who is the seme and uke even in a story that won't contain any sex, or certainly won't depict the sex, but to attach personality and physical characteristics to these preferences in sexual position.
You would only ask of a heterosexual couple "so who's the dom and who's the sub?" if you were invited to ask questions about their sex life as a couple involved in S&M. But as a lesbian, I have literally been asked in casual conversation several times, "who's the butch one?" Which het people expect to be rewarded with by an answer of "oh, it's Shaz" because Shaz is NB and has short hair and wears more androgynous clothes. They get very frustrated and confused when I say jokingly, "oh, that's me. I build all the DIY furniture and play all the shooty games, Shaz loves shopping and makeup and things." because I look "girly"; I have long hair, I'm soft-spoken and I like pretty dresses and bright colours. Then of course I have to explain to them that there isn't really always a "butch one" and a "femme one" in a lesbian relationship. But there's this assumption that there must be a masculine, dominant one who "penetrates" and a submissive, feminine one who "gets penetrated" that's frankly pretty tiresome and invasive. Can you imagine going up to a het couple and asking "So who wears the pants?" or "Sooo... she seems pretty bossy and tough, does she peg you?"
This fictional trope that queer relationships need to be forced into having the weird, uneven power dynamics of patriarchal heterosexual relationships, and that it must not only apply to the sex lives of these fictional characters, but their entire presentation and personality.
"Tops" are big, dark (and by the way while we're on this subject, it's also pretty racist that the darker skinned dude is always the scary, barbaric "top", also makes me wonder if my partner is seen as the "butch" due to being south asian while I'm white), financially powerful, controlling and assertive. (traditionally seen as masculine traits).
"Bottoms" are small, light, meek, passive and need looking after. (traditionally seen as feminine traits).
It's clearly not just treated as a relationship label by the people creating and consuming these kinds of works. The BL meme posted by Tapas also encourages this idea. If it was purely a "relationship label" that applied only in the bedroom, like being a dom or a sub, why would that be the top thing on the meme? Like the first thing you're encouraged to decide is, "am I making a top or a bottom?" which clearly suggests this is a big decision that will impact all the rest of the character creation process. ie. if the character likes penetrating best, he will be active, strong, masculine, controlling, but if he likes to be penetrated best, he will be gentle, passive, vacillating and feminine.
I think it'd be pretty disingenuous if fujoshi types said seriously that the first thing they think about when creating heterosexual characters is whether they're a dom or a sub, and that they base the entire character's personality and appearance around being a dom or a sub, especially in the context of non-sexual Romance stories. Of course, this is partially because in het romance, it's assumed that the man is the top and the woman is the bottom by default... but that's the issue here, why does a queer couple NEED to be labelled with a masculine and feminine role that covers not just our bedroom antics but our entire relationship dynamic and our personalities and physical presentation? Why is that trope so pervasive in BL specifically? Why is this "relationship label" only being applied to queer couples? I don't think it's asking too much for the BL fans and creators of the world to do a little self-examination of their fetishisation of certain types of queer people.