Red Hat makes a Linux distribution. It was not their choice whether to be charitable or not, they are the recipients of the initial charity of the Linux founders and are bound by contract to continuing making their code open as a price for building on open code.

If Red Hat could change the license, would they? Maybe. (Not now but you can’t say the leadership won’t make that decision during financial crisis).

But there's more to a Linux distribution than just the kernel, and a lot of the other pieces aren't copyleft, so Red Hat can change the license, but they choose to keep them open source anyway.