You're missing a category. Lots of big, non-IT-centric enterprises SHOULD contribute to OSS projects because they're already using and extending them (whether it violates the license or not) and it really, really pisses off corporate developers when they're told they can't release internal code into the wild. Totally demotivating.
The keyword being non-IT-centric. IT projects, OSS or not, should not be a business focus in that case. Best Buy's IdeaX (http://bbyidx.com/) would be one of the first things that comes to mind as an example in this category.
I know where you're coming from with that comment but I don't necessarily agree. Each year, information systems become a bigger and bigger differentiator on the business side and should be a business focus.
Maybe when you said "IT projects" you meant projects where IT is the customer, and if so I still don't agree. These tend to be "luxury" projects for a lot of companies and, as such, the developers tend to have a lot more decision making authority than on business projects. Letting them open source their code or contribute to existing OSS projects is a huge motivator (especially if the OSSed project gets popular and the company starts reaping the benefits of additional contributors).
I'm not sure I've ever heard anybody complain about not being able to get their code away. Have there been any studies which show that a majority of developers wish they could give their code away?
Not that I know of, but anecdotal evidence about satisfaction aside, think of the economic incentives.
If I write a patch that improves an open-source product to better serve the needs of my employers, and I don't release it, the only benefit I receive is praise and/or bonuses (yeah, right) from that employer. It really isn't much of a big deal, I'm just doing my job. As the business folk would say it doesn't really "move the needle" of my personal career. If, on the other hand, I do the same work and I release it back into the wild, assuming my patch isn't stupid, I gain some respect from people who are likely to help me in the future, and more importantly, I gain a way to prove that I know how to use and modify open source software at my next interview. I raise my market value.
Now, I can see why an employer might object to an employee releasing those changes... but for the employee, getting to publicly release code has some rather large economic advantages vs. just writing and using the same code internally.
Also, if you maintain your own, modified version of something, it can get pretty hairy as it starts to diverge from the main open source one. That divergence can cost time and money.
