The only argument that can support a hate speech law is one that blocks "incitement of violence" similar to the idea that shouting fire in a crowded theater is an action you took to harm people, not expressing an idea. If the law blocks someone from saying "I hate Christian/Muslim people" then the law is wrong and over-reaching - that should be covered under freedom of speech and freedom of expression. If the law stops you from saying, "Join with me! Let's go round up Christians/Muslims and string them up!" then the law is reasonable and justified.
The fact is that an ad company isn't a government organization and can make whatever rules about content that they want. It doesn't make or enforce laws. If they want to use a "3rd party" blah blah blah, then that's their choice.