It would be a good move for Scotland to establish laws preventing shops from keeping bettas in, and selling them in, tiny bowls. I would welcome such a law

One of my lfs has had a sign up for years saying "We do not sell goldfish bowls or small tanks".
Like Azule pointed out, they kind of lost the plot a little bit with that statement.
As for obesity in anything smaller than a 5 gallon, that bits garbage. I would welcome them into my home and challenge them to find an obese betta in any of my tanks. The obesity they are talking about probably comes from overfeeding crappy flake food, instead of listing specific betta pellets.
I believe there would be a far greater case for them to be fighting against keeping goldfish in bowls. It is likely that there are more people with a single goldfish in an unfiltered 2 gallon bowl than there are people who keep bettas that way. The health inplications are far worse for the goldfish.
Its a shame that what could be a good move is flawed by a very weak argument. Its too generalised, especially the bit about they will "irritate White clouds". They seem to have missed the fact that the temperature requirements for white clouds is at the other end of the spectrum.
They will need to provide far better evidence of obesity and general animal cruelty if they are to suceed. Perhaps they should trawl the internet and contact every breeder/keeper for opinions.