I reluctantly agree with the OP to some extent. Posted hours are posted hours and should be honored.
But I've been on the other side of the counter in many instances where (as has been mentioned) someone will slip in the store at the last minute declaring, "I know exactly what I need!" and then proceed to shop for 30 minutes or more past closing.
It's rude, inconsiderate and dangerous.

To the OP: it doesn't sound like you just intended to browse for a few minutes. It sounds like you were set to do some serious shopping. I've never understood how a store can be open 12 hours or more most days and folks decide to shop in those last few minutes.