IANAL, but having spent time at a big company, I can tell you the issue here is clarity of IP ownership.
Of course employees should be able to do what they want on their own time, but employers usually feel entitled to the fruits of things done on company time, machines, IDE's, and in general related to company work.
In this context ambiguity -- where an employee commits something of potentially ambiguous ownership to an open-source project -- is bad. Even very well-meaning companies have to manage to this issue.
I'd love to see any examples of written policies on this that manage to strike a good balance and encourage open-source contributions by employees.
