Hall: IBM, Red Hat and Free Software: An old maddog’s view

We've been here before. Whether what RH is doing would be considered an "additional restriction" or "a customer of RH using their freedom of contract to negotiate away some rights in exchange for something else" is entirely context dependant.

RH saying that the customer must pay $1 million would probably be considered an additional restriction.
RH saying that they may terminate the contract is almost certainly fine. After all, RH doesn't even need to give a reason to terminate a contract, they could do it because they don't like the name of your company, or because it's Tuesday.

Ain't the law fun? What counts as "additional restriction" is determined by (case) law in your jurisdiction, not the GPL. Otherwise you could argue that the fact that RH isn't paying your internet connection to upload the sources to other people is an "additional restriction".