If you don't advertise on your brand keywords, your competitor will. We spend quiet a lot on our brand keywords to avoid competitor siphoning off our brand traffic.
Isn't this trademark violation? I'm not from the US, but years ago I used to work for a local index-style search engine (before Google ate everyone's meal), and IIRC we consulted our lawyers and came to the conclusion that this is not legal, and we didn't sell these sort of ads.
No, it is allowed. You just can't put a trademarked term in your ad copy. You can even get away with putting the trademark in your ad copy unless the brand told Google to enforce their trademark.
There are tricks to get around this as well that sometimes work. (i.e. Dynamic Keyword Insertion)