Few overlooks...
-When Tenzin first tried to recruit Airbenders, they did not join because they did not like the Air Nomad culture. They dont want to give up their hobbies, possessions, nor their current life style
-Kai joined to escape from the Sheriff, but after some torture of the Dai Li and was saved by Mako & Bolin (plus kissed by Jinora) he rethinks his life and joins the Nomads
-The Earth Kingdom citizens captured by the Earth Queen join because they current life style was horrible. The Earth Queen abused her people and poverty was everywhere. Becoming an Air Nomad was an ticket out of there
Now as well as the Earth Kingdom citizens forced into an army, Opal WAS NOT told about the Air Nomad culture. Korra just said "go to the Northern Air temple to train with the other Airbenders", nothing about a forced life style.
Korra didnt say "intense training and hard work" like Tenzin told his students. Opal's lifestyle with her family was top quality compared to the Earth Kingdom citizens. Chef cooks you fancy food, extreme secuity, and your mother owns the city
By E7 its clear Tenzins training was torture and even those who were imprisioned by the Earth Queen didnt like it
Once Opal gets a taste of Tenizens wrath err... I mean Air Nomad culture do you think she will stay?