Unlimited SMS plans are common in the US, but the 'cartel' part comes from the pricing structure:
$10/month for unlimited
$0.35/text sent (without an unlimited plan)
$0.25/text received (without an unlimited plan)
The pricing structure is there to
drive you to the unlimited model by price gouging you if you remain ala carte. They even make it difficult for you to turn off the ability to received text messages. This way when you rack up the charges from
receiving text messages, you are up-sold to the unlimited plan rather than given the option to disable receiving of text messages. [This is also rather annoying when someone spams you with a text message advert... b/c now you've just
PAID to get an advertisement that you didn't want.]