I feel like I see this more in the overseas fandom(as far as commercialization of UTAU goes, my friend and her friends seem to have no qualm about making merchandise of their Utau, and do stuff like make art books, anthologies, keychains, open booths, etc for their Utau, seems pretty fun imo)
So anyway, I think a lot of ppl against the whole monetizing a voicebank are in that mindset that UTAU is free, so voicebanks and whatnot shouldn't have prices on them, as Kiyo mentioned. I think another thing is that people can get critical (or maybe they are just being honest at times), so there may be a voicebank/character available only by purchasing, and some people may think this is irrational because the bank may be bad quality or unappealing, so they think it's a rip-off or something to charge money for that Utau. They may think the person selling them is arrogant or foolish, reminds me of art in a way. Some people think art of certain quality is not worth selling. Some people(very irritating to artists, I'll say) think art is just something of easy fun and that it's ridiculous to be charging money for it. See where it compares with Utau? Lots of people are against people doing original songs, making albums to sell, merchandise to sell, because they think Utau is just for hobby, not something a person should make money from.
There are likely other reasons, but I think those are main ones. I don't mind people selling merchandise, (I may buy some if I want/can) and if they want to sell their banks that's fine, too, if I'm not interested I won't waste time being in a huff about it or anything, it's not harming anyone.