Your quite right, but at the time and if the officers belief is such and is re-enforced by his observations such as leaning over the the other side of the car to get a wallet which looks as if the driver is fastening their seat belt what are they to do?
Who says who is right? maybe AJRFulton wasn't wearing their seat belt? maybe the officer was mistaken? that's why you have options. Bear in mind that if you contest the ticket the the onus is on the prosecution and the burden of proof is beyond reasonable doubt and thus favours the defendant. If you could provide a reasonable argument that the officers were mistaken by way of a photograph with a driver in the drivers seat and a reasonable distance from behind then you could get a favourable decision.
Let put things in perspective. £100 yes its a lot of money to have to pay if your in the right, no penalty points and not disclosable to the insurance. Go to court argue you case, have a day off work get a favourable decision (moral Victory) grea,t but still cost you £100 for loss of earnings plus a solicitor if you employ one.
I just look at things objectively and realise sometimes life deals crappy cards.