Not allowing to bid on trademark terms is standard practice. I never, ever pay affiliates who do this (and have made it very clear in my T&C as well) because these affiliates are literally stealing money from me.
Say that my product is called MumboJumbo and you bid on the keyword "mumbojumbo". Three things are happening here:
1. I myself bid on the same keyword, meaning that you're effectively competing with me for *my own* money.
2. (More importantly) - it's highly likely that my website already ranks for the term "mumbojumbo", which means that by bidding on this keyword you are stealing clicks that would have otherwise ended up on my website anyway - but for free.
3. (Most importantly) - majority of those who enter "mumbojumbo" into google's search bar already know about my company and would thus find us without your help.
It all comes down to understanding what affiliates are for. You, as an affiliate, are responsible for bringing the companies you're advertising NEW leads, and not take existing leads (who are already ready to buy) and try to have your affiliate cookie planted into their computers.
Obviously it's a whole different story if you actually provide some value and help leads convert. A good example is review sites, for instance. Review sites do indeed pick up a lot of people who have already heard about the product but they provide value by convincing these people that the product is indeed good. This is completely accepted (encouraged even). But in your case - sending people directly to the company's landing page, that is - you haven't provided any value what so ever, and thus shouldn't be paid.
Moral issues and what's wrong/right aside - it always pays to read and understand the Terms and Conditions of the people/companies you're working with, just as you wouldn't sign a contract without first reading it through. Do this and you won't have to deal with unpleasant surprises like this.
Cheers,
Bryan