For all of those trying to define the nature of monopoly power, you are respectfully wrong. There are multiple parameters to a condition of “monopoly”, and the ability to limit competitors (in this case other racing series that could compete) is one of them. The impact on the consumer is one of the most important gauges of “monopoly” power, and the amount of harm possible from that.
The plantiff’s are basing their arguments on claims of exclusionary conduct, which they say is harmful. NASCAR claims a business justification for the agreements they have created. Below is a summary from the FTC and a link to the website. Much is open to interpretation, hence the need for appeal by NASCAR:
FTC Link On Monopolization