Take another look at the original sentence:
You wanted to fix it this way:
By placing a because in front of she always watches, you subordinate what was a main clause. This method is a legal move in writing. There is a problem, however. When you attach a subordinate clause to the end of a main clause, you seldom need a comma. The comma before because is the problem with this choice.
You might want to consult the rules for fixing comma splices and fused sentences.