Why does Intel support Linux?

From everything I read, Intel is a big Linux supporter. Now they are
joining IBM and others in setting up the legal defense fund for Linux
users. I think that's great but I don't understand the business logic.
In a Windows world people are pretty much stuck on Intel iron except
for AMD which is a much smaller company. With Linux, they can migrate
away from Intel any time. So why is Intel pursuing a strategy that can
cost them market share.