I wonder whether it's necessarily unintended.
At least since Peltzman/Stigler, we've come to see regulation as a balancing of the interests of the regulated and the public. The former here we can proxy as the big soda makers.
It might not be crazy to view a ban that just catches anybody using an international standard size, 500mL, as something that imposes disproportionate costs on small producers. Big guys can spread fixed costs of this sort over lots of units, and are probably already using US standard sizes. Anybody who's gotten a good deal on a few containers of international standard metric-sized bottles winds up stuffed. At the margin, Bloomberg (or, more likely, his staffer) might have thought of this as a way of making the big guys a bit less angry about the regulation.
I'd not run the metric conversion in my head when the policy first came out; my in-the-head heuristic is 8 oz = 250mL - both are about a cup. But that was wrong: Google tells me 8 oz is 237 mL. Maybe Richard Thaler needs to nudge me into using better metric conversion heuristics.
Apologies for the brief posting hiatus; semester constraints have begun to bind.