I cannot agree more, Carl & cbg. Unfortunately, too many HS students are never taught the art of writing a thesis statement, leaving their college professors to either teach them or pass them along. Neither of which is, to *me*, an acceptable solution.
In terms of acceptable topics, there are any number of reasons that a thesis topic could be considered inappropriate. Too broad or narrow a scope, inappropriate for the course in question, too difficult to address within the requirements, triggering to too many fellow students, etc. My daughter had to choose a different thesis topic (in college) - once for being too broad in focus and the other for being too difficult to prove given the time and parameters specified. Once her topic was approved, she had to adjust the scope of her statement to her advisor's liking. That's how the game is played.
Out in the real world, you have to play by the rules. Even when and if they change.