Imagine if you gave away some old clothes to some Charity and they called you and said "Some of the socks have holes in them and we need you to come over here and fix those holes ASAP because we want to sell them in our used clothes store". What would be your reaction to that?

The expectation of payment is not for the software (which MS already has and is already using, free of charge, same as everybody else), it's for getting priority in bugfix and maintenance work, or in other words, it's for dictating other people's work rather than merelly getting the product of work they, of their own choice and in their own timings, did and gave away for free.

Free software is a social relationship, not a business relationship: the users get what they get because somebody chose to put their own time into it and is giving it out for free. Such relationship does not entitle the recipients of the goodwill of others to make demands on their time, especially if said recipients are actually profiting from what those other people gave away. If they want the right to get to use other people's time as they see fit, then they have to get into a business relationship and that's only going to happen in business terms that both parties are willing to have.

Further, nobody is stopping MS from using their own programmers to fix that problem themselves.