Because if there weren't any spell tomes for them, someone would complain that it's impossible for everyone to know those spells when *obviously* the books that teach them don't exist.

And while every PC might start out with those spells, how do we know that every NPC in the game knows them? Besides which, the PC had to learn those spells somehow, and it's not like there's only *one* tome on Tamriel for every spell in existence. The PC found tomes for them at some point in his/her backstory, and used them - but that doesn't mean the gazillions of other tomes for Healing and Flames suddenly disappear from the world. It would be like starting you off with an iron dagger in your hand and then having it be the only iron dagger on Nirn.
