i think its usually that way for local stuff. unless the client writes it or they have an ad agency. most local spots are written/produced/directed by the tv station. on the national scene usually the creative team at the ad agency writes it based on a position brief from the account planners. Then they take bids from directors/production companies.
Exactly. most production houses take care of the "prodution" part of the commercial, while Ad agencies normaly come up with the creative concept. Some production houses do everything in-house, from bidding to actual production and allow the creative process to go trough the director.
But, yeah, the creative writting part is an extra fee (about 20 to 30% of the whole fee, if you are good at it).