I think it has more to do with preserving the fiction that they're significantly more independent from commercial interests than other news media. Losing dedicated public funding would make them a commercial network that also begs for donations a few times a year, probably harming donation and removing some of the halo-effect that advertisers are looking for by "sponsoring" them (paying for ads).

In short, it'd push them farther out of the ad niche they've carved for themselves, which could have serious revenue consequences beyond that 1%.