Constant -Luton airport (or at least the company managing it) is a company run for profit. Certainly, the company realises that if they annoy customers sufficiently that they go elsewhere next time, the profits may drop, but the company is betting that the increase in trolley charges is something people will tolerate enough to avoid people moving en masse to Stansted.
If trolley usage falls 80%, the charges will be lowered pretty quickly. If trolley usage declines only 10%, then trolley profits will have increased significantly, and the increased charge will remain.
Once Joe Bloggs and family have made their booking to/from an airport, they become considered fair game for whatever money making scheme an airport may dream up. Customer satisfaction exists only to ensure future profits. As long as you keep flying from their airport rather than a rival, airport management don't really care whether you resent paying that extra pound or two.