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"Essentially, they cost nothing to the carrier in that they get transmitted as part of the control channel. "

That was true at one point, but the popularity of SMS has completely swamped the capacity of the control channels.

Care to elaborate? Doesn't my phone always have a control channel signal?

At least as far as CDMA and WCDMA go, control signalling is shared for all mobiles camped on a cell. In CDMA, while a mobile is just camping there's not a lot of traffic going between the cell and the mobile on the access and reverse access channel. The mobile registers as per the registration settings, and the idles its radio until it is paged by the cell, it originates a call, or the registration settings require it to register again. When idle, there is no signalling going back and forth on the access channel, so that bandwidth can be allocated for other things.

I'm not at all familiar with GSM, so I can't say how it works. I expect it's similar, though.

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