It seems like any company can be legally forced into spying on you, and if that isn't already the case then they can always pass another law to make it so. Thus any such promise would be essentially toothless. Probably the strongest thing they could say would be "we only spy on you to the exact extent required by law, but by law we can't even tell you what that extent is".
http://www.aclu.org/national-security/doe-v-holder