I spoke to the head of SMS.ac today. I complained about the Spam and Premium SMS charges. He seemed genuinely surprised that I was being charged for Premium SMS. He suggested that I change my profile to block incoming messages. I dutifully logged on to the site and checked every possible box that I could find to block people from sending messages to me. I'll see how it goes.
I can imagine why carriers love the service. The largest community on SMS.ac has 28,000 members. No cost for a single member to send a message, but each of the 28,000 members would pay to receive a single message sent/delivered to the group.