Your policy doesn't make any distinction between open source licenses that do grab any and all patent rights in scope; and those that don't. If your firm has no patent IP and doesn't plan to get any, fine and dandy. That's not the usual case for large firms, and you can't let every employee make legal judgments on behalf of the whole firm, that might severely affect the firm. Hence a firm rule. Fortunately, licenses which appropriate patent rights as punishment for daring to contribute are getting rarer.

Note - these clauses also seize relevant hardware patents, they make no distinction between hardware and software patents. Will a court say otherwise? Pardon me if I don't bet my company on that question.

Discouraging work for anyone else at all can have other motives of course. Businesses often want legal rights to all inventions an employee may come up with during employment even if unrelated to their business in any way, for example.