Perhaps they rely on some flawed signals to make their decisions such how many positive reviews you have (the buyer) vs seller's reviews(i.e who is more likely to lie).

There could also be bad faith from Airbnb as some comments frame it but I believe most of the time it's just a tricky business. Check ebay and you see the same issues or even worse.

I'd much rather have one legit review removed for every 20 fake one, rather than have to wade through 20 fake reviews just to find a single authentic one.