People are inclined to dump them if there is no nearby place to leave them. That is a trolley management issue. It should be the responsibility of the company to manage their trolleys anyway, not expect the shopper to act as unpaid employee putting them back.
The cost of freezing wheels may be extra on the top of the initial purchase price, but is insignificant over the life of a trolley: and if it isn't cheaper than losing or retrieving the odd one, then write the lost trolley cost off as a business expense.
If folk get annoyed at not being able to take them off the grounds/car park then one has to wonder how they realised it, and why they were trying to take them in the first place.
Yes is is too much trouble to have to find a coin and use it to get a trolley. Which is why one shops elsewhere where the customer is valued rather than considered no more value than a resource to solve the shop's problems.