I'll tell you why.
Those glass screen protectors will probably be the thinnest pieces of trash, they won't adhere to the screen as nicely as the £10 ones or be anywhere as durable.
The clear case from Apple at £49 - Whilst I agree it's a little high in price for what it is but it will not yellow at all for the duration the owner has it. That one you have, I can guarantee will yellow and look ugly after a while. Also, those magsafe rings are horrible compared to the real deal.
Each to their own and it's up to them how they want to protect their £1000+ devices.
It does not make them a mug.
EDIT: I have just read your comment that, that clear case has no yellowing after a year of use so the jury's out on that one