The only reason they are charging you is pure profit for the phone company.
Cell-Phone Bills: Is Text-Messaging Too Expensive? - TIME
What's most amazing about the texting craze is just how inexpensive it is for mobile carriers to provide this wildly popular service. SMS messages are not only extremely short (maxing out at 160 characters), but they also cleverly exploit today's digital phone networks, leveraging transmission channels between phone and cell tower that were originally designed to coordinate voice calls. "They cost the mobile carriers so little that you could argue that they're free," says Collins.
You don't have to be a Wall Street analyst to do the quick math: with a carrier cost of one-third of a penny, when a customer pays 15 cents to send a message, 98% of that 15 cents is pure profit. (Of course, you already knew that in your gut; that's why your stomach turns every time you examine your cell-phone bill.)