I have checked the MOT manual and am rather shocked to find that the operation of the inertia or deceleration operated locking mechanism is not actually tested, which makes that part of the MOT rather pointless. I would prefer my MOT tester to pull them to check them properly, whether required to or not. Safety is what matters, and whoever wrote certain parts of the MOT testers manual seems not to be very clued up about that.
It sounds like you think the MOT is not fit for purpose?
You need to consider what the MOT is trying to do and what is reasonable to check. I personally think we all should be responsible for our own safety, the MOT is a good basic check of roadworthyness (on the day of the test at any rate). Who is really responsible for the safe operation of your seatbelts? or any other part of your car?
You can't really blame the goverment, or the garage (assuming the MOT was correctly done). the reality is that you could inspect a tyre, and 2 miles later it could explode and cause an acident. The test is what it is, it has limitations, but should in most cases identify safety related faults.