Yes it's your money. You could spend it all on Jan 1 and quit on Jan 2 without ever actually having anything deducted from a paycheck. If you contributed all year, but didn't use the money, you would lose the entire balance (less whatever they allow to roll over), and if you quit with a large balance (that all came out of your paychecks) you would forfeit that money. This is just the other side of that coin.
Yes it's ok to take advantage of that IMO. They wouldn't have any problem forcing you to forfeit unused funds at the end of the year or upon you quitting as per the rules, so I don't see any problem with you using the full balance before quitting, exactly as per the rules.
Enjoy your free FSA bonus.