This is relatively standard for mattresses. Too many regulations surrounding their resale so many places will just ask you to "pretty please" donate it so they can write it off as a donation.

I find it odd that one item has so many regulations but others that I consider in the same category don't. I can return towels and bathing suits for example. Why are mattresses so heavily regulated like this but other items with similar surfaces for problem are not?

Price is probably a factor. Mattresses are expensive, making it more worth to screw people over with them, and making it worse for the person who got screwed.

1) regulations rarely make that much sense, but also

2) bed bugs

Specifically, bed bugs are very, very hard to kill once they infest something like a mattress. When they are dormant, they're almost invisible and almost indestructible. There is no effective way to remove an infestation from a mattress that doesn't either destroy the mattress or involve very toxic and/or illegal chemicals. (DDT was really the only effective pesticide for them, but that turned out to also really fuck up ecosystems, so we banned it)

Those items are probably never resold. They just throw them.

Mattresses are big. They stand out when left on the side of the road.