Yes, I’ve been involved in monograph acquisitions for an academic library, though not for a public library. The only difference, however, is that academic libraries add more of their donations to the collection, because they are trying to be more comprehensive and because they are happy to add things just-in-case someone might need them in the future.
Most libraries accept donations, but generally only on the understanding that they are free to add them to the collection or dispose of them as they see fit. If you add it to the collection, you have to catalogue it and find space on the shelves for it. (And shelf space is not free). With public libraries, there is the added consideration that they don’t want stuff on the shelves that never gets read, because it makes the users feel that the library is less relevant to their needs. (This probably ought to be a consideration for academic libraries, but they have more of a captive clientele).
There’s an exception to the general rule, which is when a person donates a collection of really special material (like grandad’s collection of medieval Tibetan manuscripts), they can insist on the collection being retained by the library. However, even then the library has to decide whether it belongs in the collection: it’s quite likely that in that case they’d reject it, but say, “Here are some libraries that specialise in Asian materials: they’d be much more suitable for giving this wonderful collection to.”
As far a considering donations of politically slanted materials is concerned, I would think that’s a point in their favour, as long as,
(1) the collection remained balanced on the political issues after including the donation, and
(2) there was likely to be some interest among the library’s clientele in reading up on the issues.
But I still would not automatically accept it.