I think the argument is that, if they expected to receive high-quality contributions, then they'd be willing to take the risk of competitors using their software to compete with them, which an open-source license would allow. It usually doesn't work out that way; with a strong copyleft license, your competitors are just doing free R&D improving your own product, unless they can convince your customers that they know more about the product than the guys who wrote it in the first place. But that's usually the fear.

On the other hand, if they don't expect people outside their company to know C++ well enough to contribute usefully, they probably shouldn't expect people outside their company to be able to compete with them either.

Really, though, the reason to go open-source is because it benefits your customers, not because you get contributions, although you might. (This logic is unconvincing if you fear they'll stop being your customers, of course.)