You are absolutely correct. You shouldn't keep those fish in a 30g, not even a 2" Tang or Trigger. The other people that are doing this, well, they don't really care about the well being of the fish.
Or the people doing this are simply not aware of the requirements of the fish
Tangs are very active swimmers, and need swim space. The general accepted minimum tank size for a Tang is 4', but bigger is better.
In addition to swim space, some fish are territorial, and bigger tanks allow them to have their territory.