I didn't think that simply selling their own rooms at a fixed price would be called "price fixing".
This is what used to be known as retail price maintenance. IHG sets its prices and persuades any retailers to follow its lead.
Essentially it eliminates competition among agents, and between the agents and IHG. In essence the Expedias
et al are simply extensions of the IHG booking engine.
The smaller "rebel" agents such as Skoosh get frozen out if they don't follow the line: and hotels are strongly discouraged from selling rooms themselves, or through intermediaries, at prices lower than those available through the IHG reservation system.