The rationale behind this question is because AppEngine is free until certain limit and I found it to be a good option for small businesses (especially in developing countries) that can not afford a webhosting and does not get alot of traffic to their website either. My question is can we use it to host our customer's website that runs small businesses? Is there any legal agreement saying that we are not allowed to use AppEngine for this purpose?
I'm not aware of any clause from Google that prevents you from doing so. However, I would advice that you caution your clients about the trade off:
Who owns AppEngine? Why not ask them this question directly? It is easier (and cheaper) to ask than do something based on a false assumptions.