The only problem I see, is the difference between telling someone a betta CAN live in 0.5 gallons of water, and saying it SHOULD live in 0.5 gallons of water.
Just because they can in the wild, does not mean that we shouldn't strive to give them a better habitat while they are in captivity. I see nothing wrong with guiding people towards a larger enclosure, such as 2.5 - 5 gallons, as opposed to telling them that a 1/2 gallon is perfectly suitable.
There are no cons to a large enclosure, however there are many to a small 1/2 gallon enclosure, if someone isn't very keen on cleanliness of the enclosure. And most general joe schmoe betta keepers that buy them for their kids, are not all that keen on daily water changes. Something a 1/2 gallon enclosure would require.
So getting all upset when someone says a betta needs bigger than a 1/2 gallon enclosure is confusing to me. Yes, they can survive in it with proper care, but nothing wrong with going bigger, so that the betta can have more room to be comfortable.
While some betta's do live in the wild in area's not much bigger than a 1/2 to 1 gallon of water, it's also natural and out in the wild, with clean rain and fresh air. Not an enclosed plastic container with stagnant water.
And when the rainy season comes, that 1/2 gallon swells to many gallons, not including all the betta's whose natural habitats are large rice paddy's and small streams.
Nothing wrong with educating people that while they can survive in a 1/2 gallon of water, going bigger can only be an enhancement on the life of the fish.