Admittedly, really we shouldn't need laws like this - it should be a common sense thing. However, lets face it - common sense just isn't that common, anywhere.
For all the talk of badly run pet shops, all the ones I go to, and have been to, are clearly staying in business for a reason: e.g. convenience, pricing, advice, stock held, offers, atmosphere, whatever. That reason is (sadly) not always how well they treat animals.
A shop that stays in business generally does so becuase of cashflow or profits, regardless of how they get sales. They might have very little (ethically) to say for themselves, they may well be prepared to sell any fish to any buyer, but they still make the $$ and therefore stay in business.
I'm not in any way anti-capitalist or whatever, but think of Nestle and the baby-milk scandal, or "blood diamond" scandal, or non rug-mark rugs, the list is nearly endless. The companies involved generally do as well, if not better than more "ethical" companies. Most people do want little independent shops, butchers, fishmongers, etc, to keep going, plenty of people do tut at Walmart; but they still shop at there.
Same for pet shops who are less than responsible: they continue simply because people still shop there.