I think the opposite is true: still quite high costs are the cause of SMS spam. If the carriers did not profit from this, they would have destroyed SMS spam as a phenomenon on the same day.
AT&T, Verizon, US Cellular and T-Mobile have all imposed fees on Application to Person SMSes, so they definitely are making more money off this: https://www.plivo.com/blog/a2p-10dlc/