Having a permissive open source policy is important if a company wants to recruit truly stellar programmers. Or put another way: great programmers will be less inclined to work for you if you have a restrictive open source policy because being involved in open source projects is one of the best ways for a programmer to increase his market value.
Traditional methods for measuring programming ability are ineffective
The job market for programmers, especially the top programmers, is notoriously inefficient. This inefficiency is due to employers lacking good methods for evaluating programmers. The standard techniques used to evaluate programmers -- resumes, on-the-spot coding questions, take-home projects -- are at best crude approximations of a programmer's ability, and none of them will be indicators of the truly visionary people. Sure, there are other indicators like being involved in successful companies or having past impressive titles, but those are still indirect indicators of programming ability.
If you're a programmer, this difficulty in measuring your skill means its really difficult to make a potential employer's perceived value of you match your actual value. Top programmers aren't differentiated from the next tier of programmers and get badly mispriced in the market. Top programmers need better mechanisms to communicate their value so that they can be priced more fairly in the market.