There's trouble with premium services. Some unscrupulous services send messages into the blue which (when responded to) lead to premium charges.
And don't think it's all so rosy even when that doesn't happen. The main services using premium SMS are (indeed) porn offerings. Dating services, naughty pictures over MMS, etc.
Other prime services are such shady dealings as horoscopes, pyramid schemes, and other things that sully the name of the system.
Not to forget the betting systems where you get (at 1.50 or so per message) dozens of messages you have to respond to with some answer and in the end hope to win the grand prize.
So you see, there's little difference between 0900 numbers and premium SMS services. There's tons of smut in both and for the same reason: smut sells.