I went to IBM(India) interview sometime in 2011, after leaving GlusterFS (Red Hat). Interview went well, during final call with management. I was asked to stop working on Open Source during weekends or off-hours even though the IBM project and my Open Source work has nothing in common. I said, "I thought, IBM support Open Source right?" his response, "Yes, but that's another team" I decided to call-off the interview…
I am currently working for IBM and I'm tied. I had to supress my commercial project once I joined big blue. What I have in contract is - if you want to open a company, you need IBM's permission first. And it's just frustrating as they try to block it for as long as possibe even if you're not doing IT in your private little business.
I am pretty sure this is the standard practice at all large companies, at least in the US. Small companies may just not care too much, but even at a small company if your management notices you might have to choose between that and your day coding. I wish it was not like this, but to me this is at least somewhat justifiable.
Much worse is the desire of most employers to control everything you do, including your work on open source project off hours. Want to fix coordinate computation for an open source satellite sim? Call the lawyers first. Lead a robotics club at a high school? Check with the management. IMO many employees do it anyway and hope to not get called on this, but this is formally going against the contract.