They tell you not to, but don't really enforce it. What they don't want is everyone loading up their trucks with tons of tanks and running the shop low on them. We always kept a spare tank just in case we get a bad one (bad o-ring, etc). They aren't at the drive-thru most of the time and don't care the rest of the time. Never said a word to us about going over the tank limit.
They also don't want people herping and derping 16 tanks and having them fly through someone else's windshield behind you too.
Like most things on Bonaire, the real rule underneath the rule is "don't be a dick".
I like the "take one spare tank" advice. It hadn't occurred to me that if there's a bad o-ring, we can't replace it at the dive site because we don't bring anything of value, like o-ring picks or any other tools. I can't imagine they would notice us taking ONE spare.