I previously worked for another LCC (i.e. not RYR), but general policy is to not use the APU for air conditioning while on the ground in a turnaround. The APU uses fuel, so of course it is cheaper to have it turned off. It sounds as though RYR policy is to only use the APU for engine start. Our airline allowed us to use the APU if things got too hot and cool the cabin; down to the crew to make the decision on the day. In my current airline, there is absolutely no problem in using the APU to cool the cabin.
If the aircraft you are on pushes back and starts its engines normally then it will have a working APU and consequently could have used that for cooling the aircraft. Normally if the aircraft starts its engines on the stand it is probably using an external air cart and the APU is inop. Most big airports will have air-conditioning systems on the stand and these can be plugged into the underside of the aircraft and cool air pumped into the cabin (which is better than the APU) but this is of course something which is charged to the airline.
So, it is all about money and looking after passenger comfort. Some care, others obviously do not.