Ya can't lock a strap, I use these and have been for years. Been using a pad lock on a pair of them for years. The pair are mounted on my trailer which lives in our carriage house. The OP is asking a valid question just as long as he isn't a poser which some here have blatantly accused him of.
On our long tours they are kept locked for two reasons. Ya never know what goes on at our once every fifth day motel parking lot, and you never know who's gonna wander into your campsite.
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