A retailer is given a Price List/Catalog every year and it contains some rules for selling discs. I believe there is language that says if you sell Innova Discs, you agree to abide by this contract.
So yes, there is a contract.
Next, I have no direct legal knowledge and I am not a lawyer, but the anti-trust/monopoly case against Intel went along lines that are very similar.
When Intel started raising prices and refusing supply to any business that used AMD processors because AMD a competitor and Intel began to refuse to honor past relationships with AMD because of competition with AMD, they were sued for anti-competitive behavior and ultimately lost.
You cannot refuse to sell to someone because you see them as a competitor.
It's called anti-competitive behavior. You also can't price preferentially (below cost) or too high to drive someone out of business.
It's difficult to prove and Companies usually settle out of court, but I'm fairly sure that it would be illegal for Innova to stop selling discs to competitors for the purpose of hurting their competitor's business.