customers to leave their trolleys at the parcel and trolley deposit sections regardless of where they want to go. No where do they state that it's only for customers buying stuff and not those who have already purchased and wish to go around other sections!
Assuming the story of the OP's to be completely true, then he should have been aware of what times exactly the lockers close. Generally all places have their opening and closing times stated somewhere, so it pays to be informed ahead.
I wonder though why did the customer service guy get all rude? Was there any provocation?