Do they forbid using the code, or incorporating the code into their own works? I hear of places forbidding the use of GPL code (even in-house) where they use proprietary software like Microsoft Word with no worries.
No, they might not. Just like Microsoft, they might demand that you don’t release your plugins. Addidionaly, GNU software gives you the alternative of releasing your plugins. But you don’t have to, and they can’t make you do it.
Corporations choosing to not use GPL software mostly don't know enough about it, to know it's ok. So they ban it outright.