Well, in my particular case the main issue is that we’re a university science library. Our collection includes works on agriculture so yes, a book on growing pot would be a better use of our resources than even the finest romance novel.
That’s the problem with the idea that a library should put any and all donations on the shelves. Most donations are made by people who are just trying to get rid of books they don’t want anymore. A library isn’t and should not be a homeless shelter for books with no place else to go.
I doubt you’d find a public library in the US that didn’t carry romance novels or other genre fiction. This wasn’t always the case, but it has been for decades. Donations of popular fiction will generally go to a fundraising book sale rather than on the shelves though, not because modern libraries object to popular fiction but because the kinds of novels people are most likely to donate are in poor condition, are already owned by the library, or are titles that were popular years ago. A donation of The DaVinci Code would not at this point be much use to a public library. (I just checked online and my local public already owns three copies, none of which are checked out.)
Part of the reason for their financial success is that some romance fans buy Harlequins like other people buy magazines. They get a new one every time they go grocery shopping. Every so often these avid romance readers need to clear out part of their collection, but while there’s high demand for new romance novels interest in old titles is minimal. Most secondhand book stores won’t touch them. I mentioned romance novels rather than some other type of popular fiction not because I hold them in special contempt, but because of the sheer quantity that people try to dump on libraries.
If public libraries accepted all the old Harlequins that people try to give them then they’d soon be stacking them up to the ceiling…and no one would be checking any of them out.