Well, there's the rainbows and puppydogs ethics where we do our best not to harm anything for any reason. But as for the culture we ACTUALLY live in...
We kill and eat animals. A lot of animals. And our attempts to make sure they die humanely for that purpose, regularly fall short. We capture and kill wild animals to cut them up and see how they work, or to stuff them in zoos or glass boxes for our amusement. We may have any number of justifications for this, but they don't make the end result for these creatures any less dire. It seems the only obligations we (generally) cling to in their defense, is to prevent "needless cruelty."
To that end, yes it is reasonable for LFS's to try to establish that the hobbyists they sell to are well-intentioned, and properly equipped to care for the livestock they're buying.
Where the lines get drawn between a "reasonable thing to do" and "required by law" nearly always ends up as a cost/benefit question:
- We're likely willing to require that store owners ask a few basic suitability questions, but in that case the customer is free to lie, and all we'll probably accomplish is to slap a sticker on each holding bin, such as "Royal Gramma, Salinity: 1.021 - 1.025, minimum tank size: 30 gallons" and the LFS is off the hook once you swear your tank is at least 30 gallons in size. Is this going to make enough of a difference?
- Are we really willing to go through (and pay for) a process to have our aquariums tested, inspected, and certified on a regular basis, with a license required to buy aquarium livestock? This will probably do a lot to improve the captive conditions for livestock, but it'll also probably come close to killing the whole industry.