As you say....trolleys still need to be collected.....
less people using trolleys = less trolley collectors = less salary to be paid out
no people using trolleys at all = no salaries to be paid out at all = no trolleys to be bought/repaired = zero cost.
if you think logically - it makes sense to charge for a trolley as it will save/make more money than it will lose by the odd person flying out of another airport such as EMA (major competitor?
)
Just watch other airports follow BHX example.......