Some companies are concerned about the 'viral' nature of the GPL in particular (some suit read an article about open source that talked about the GPL, and now 'open source' == GPL in his head) There are still many unresolved questions about the GPL in the US, as I'm aware it's only been rarely if ever tested in most jurisdictions in an actual court of law.
Personally, I expect to be compensated for my time and effort. This needn't be in money -- I release free software as a 'gift' for the community because I (and most of us) have received many such gifts in kind (Indeed, almost all the software I use, from the kernel down to the tiniest little nifty script) was a 'gift' to me by other members of the community). A commercial interest, on the other hand, will have to find some other way to compensate me for my work, as they (typically) are not part of the 'community' that has already compensated me for my time. Cash works well.