In 15000 miles and 16 months it is feasible your tracking could have gone out of alignment due to any knocks the wheels have taken from potholes, kerbing etc. If they charge you or not is down to any company policies they have on it and pure goodwill.
As far as half the tread being half-worn on the rest of the tyre, partly depends on the rubber compound of the tyre, softer rubber grips the road better (so are safer) but wears out quicker, more expensive (and sportier) tyres like continental and pirelli tend to use softer rubber compounds for the better grip. Cheap tyres tend to last better as they use harder rubber compounds but don't grip as well particularly in wet weather, also, I've noticed the cheap ones tend to crack on the sidewalls earlier due to UV light damage earlier than expensive ones.
Wear also depends on driving style, conditions and vehicle (power, weight, tyre pressures, steering and suspension set up), so it's impossible to say if it's reasonable or fair, due to too many variables, I wouldn't consider it as outwith the realms of acceptability. As front tyres wear out quicker than rears on front wheel drive cars many people have them changed round to even up the wear.
Sorry, there isn't really a definitive answer, every case is different. Good luck.