Because the facilities cost money to maintain as others have made reference to.
Need funds for up keep
The persons that are on duty as instructors or lifeguards are not volunteers they are being paid for their services.
The same reason the City of Tucson charges to use the pool, or to reserve a Ramada for a party. Someone has to do upkeep. The maintenance crew and staff get paid for what they do, and charging a nominal fee for use of facilities is reasonable.
To help with up keep.
Who cleans the park? Does maintaining? These people have to get paid. Just like the pool pays the lifegaurds.
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