Open Source doesn't mean free. You can have Open Source that is commercial as well, and for which you need to pay a fee to get a licence. There are many Linux distributions that were or are commercial and require to purchase a licence to use them. Many corporations have provided very valuable code to Linux and other Open Source projects, and probably now even more than what individuals are able to provide.

What corporations are afraid of is that their Intellectual Property will be stolen. If they have a way to ensure that their IP would not be stolen, most companies would endorse Open Source without a problem -- Microsoft included.