I'm not sure why you would split centaurs and mermaids. A centaur is literally a human on a horse and a mermaid is literally a human on a fish.
Sure, but the emphasis is... different? When it comes to a centaur, it's undeniably horse. The sexual parts are generally going to be equine in nature, too, which is a huge part of it when it comes to what defines bestiality, at least by Amazon's standards. When you look at a centaur, you know that you're looking at something that is part horse. Sure, there are different breeds of horses, but they're all pretty close to being the same. A horse is a horse in the same way that a dog is a dog.
But when it comes to mermaids - well, it's less like "hey, this person is half horse and half man" and more "hey, this person is half human and half undefined aquatic animal." Is the tail more like a goldfish's or a shark's? An eel's? Is it serpentine? Can you associate any specific species with the lower half, or is it simply fishlike in the same way that dragons are very often reptilian (but, again, not directly connected to any specific species)? "Fish," at least with how I used it, is
far more varied than "horse." Horse refers to a specific animal while fish refers to an entire group of unique animals. There's also the fact that mermaids are generally going to be human when it comes to sexual anatomy. That makes a big difference. Dragons might not be human when it comes to that aspect, but as long as they're not like an animal in that regard, they're fine.
Then there's also just cultural acceptance. At least here in the US, people are going to be
far more comfortable with a mermaid than a centaur when it comes to romance/erotica. Centaurs are seen as much more bestial - they're just as close to being animals as they are close to being humans. They are often as wild as actual horses are. Sure, there are tales featuring humans with centaurs, but not that many and it's usually portrayed as taboo. Mermaids are basically just seen as beautiful humans who just so happen to have a fishlike lower half, and there have been many stories romanticizing them and normalizing it.
In other words, mermaids are like missionary sex with the lights off and under the covers, and centaurs are more like BDSM to your average person. Except, given how BDSM is allowed on Amazon while centaur erotica is not (any centaur erotica you can find simply hasn't been reported), it could probably be compared to a far more taboo fetish.
Personally, I agree that they're basically the same and should be treated as such. But, looking at how they are viewed and treated on different platforms, they are on completely different levels.
Won't this also make tentacles fun pretty much bestiality? I mean it's basically people getting fucked by octopus-y things. Damn it, there went my Kraken x mermaid story...
Basically the same as what I just got done explaining. Are they specifically like octopus tentacles? Are they specifically like squid tentacles? If so, congratulations, you have bestiality in the eyes of Amazon, Patreon, Royal Road, etc. Are they not directly related to any real-world species (like most lewd tentacles)? Then congratulations, you don't have bestiality. It is extremely easy to make a tentacle nothing like any real-world tentacle.
As long as you can't look at something and immediately make the connection between it and a specific animal, it's probably not bestiality.