a) She enforced the rule incorrectly. There was nothing close to a foot fault on that serve, as the replay clearly showed.
b) Foot faults happen all the time, but are almost never called. You don't start calling something like that on a second serve, one point from match point. If you are going to call a foot fault in that spot you'd better be darn sure it is the right call, and it was not.
It looked like Williams was going to lose, anyway, as she was down a set and seemed to be struggling, but the officials should not impose themselves on a sporting event on that way.
Looking at the smug look on the lineswoman’s face, it seemed clear, she wanted to put her stamp on the match, with that incorrect call. It was disgraceful (as was Williams’ reaction). She should never officiate another match.