Don't see how that can be enforced unless you have luggage in the hold? Just get off when it stops at Heathrow! Obviously a different matter "starting late" in the opposite direction.
If you don't have any luggage in the lockers, yes it works. In most cases though, if you are going to the Airport, you will have an item of luggage which requires to be in the locker.
It's marked based pricing at the end of the day. Public transport is often still the cheapest option unless you are travelling in a big group, for a very short trip (where parking would be cheap) or departing/arriving at a time of day when you'd need a hotel to use public transport.
Public transport isn't always the cheapest option, especially when you consider that people don't tend to see the full cost of using a car, they see the immediate costs which come with that journey, in the case of driving to the Airport, they would see the cost as a bit of fuel and the parking. Even if the car is more expensive, if the cost difference is low, people will just go for the car for the convenience. You've also got to consider family/friends giving lifts as again, that will be cheaper than public transport. Public Transport has to be cheap to be attractive and the fees the Airport push onto companies makes it not attractive and is rather anti competitive as the high fees mean that there are limited companies willing to pay the fees to serve the Airport. Bristol for example could have a good few additional buses there if the fees weren't so high, Edinburgh similarly wouldn't have companies serving Ingliston Park and Ride to avoid paying the silly fees, they would just serve the Airport.
Airports provide infrastructure, are they not entitled to payment for the use of that infrastructure…?
That depends if the infrastructure is good enough to be paid for. Edinburgh Airport is just a few random bus shelters, that's all that bus companies get. Whoopie doo. Bristol the bus station is nothing more than a car park with a few barriers in it and a portacabin for waiting (though be willing to be the last on the bus if you wait in here, and possibly left behind if the bus is full).
I'd add as well, charging a normal departure fee is completely justified and this is how almost all bus stations in the UK cover their cost of operating. The issue is the extremely high departure fee or the access charge, however it is charged, which makes fares significantly higher than they need to be. For a tram fare to jump by £5.50 for a single stop is just bonkers and suggests that the fees are astronomical.
Sadly both airports I have found to be extremely secretive over their fees so it's hard information to get hold of, if anyone can give the actual fees, it would be very helpful.
