why wouldn't it be? you can take pictures of people in public and post them online legally in the u.s. a better question here, is what are you planning to do with it, and is it ethical and moral?

Why wouldn't it be?
You can take pictures of people in public and post them online legally in the U.S..

A better question around this topic is:
"What are you planning to do with an open face recognition database of public people/celebrities?"

And an even better question for this topic:
"Are your plans involving creating an open face recognition database of public people/celebrities ethical and moral?"