Legality of Book Stuffing?

[Bubo]

I just had someone tell me how he gets new business. My first gut reaction was to tell him that his method was wrong and illegal, and then I paused and thought about it for a moment. I thought and thought and thought, and then realized that I can't recall anything besides location rules that would oppose this:

He goes to book stores (mainly used book stores) and places business cards inside of how-to books related to his work.

I would imagine that this would violate the anti-solicitation rule that a lot of the chain book stores have (he hits those anyways), but many non-chain book stores do not have similiar rules. Does anyone know of any laws that would oppose this activity?