Have you ever heard of this happening? For Westlaw/ Lexis to do this would be really silly. They are trying to get you "hooked" on their product, and if you do they have a potentially life-long customer. I can't imagine them busting a student, considering their fierce competition with each other and what is at stake.
They're trying to get you hooked, but they're also trying to get the firm you're working for to pay their astronomical fees. I think they figure your free using during school is more than enough to pull you in; they would rather have your firm pay for searches. Also, its one thing to give you free access for school, its another thing when you're billing clients and looking to make a profit.