If they send you a C&D letter, how could you argue they don't protect their TM. That's what the purpose of a C&D letter is. As the owner of the TM, they have the full right to decide what parties they think are potentially damaging their TM and protect against them. If they allow 20 sites to operate without incident and come after yours, well, they have the right to do that, because you have absolutely no rights to their TM, simple as that. I can't break into someone's house and make an argument for ownership of their goods because they didn't protect them well enough. Whether you get away with it or not, theft is still theft.