Or is it simply that those are the plans that don't offer text messaging? In which case, situations like spamming black pages at remote fax machines come to mind. And all the legal ramifications involved.
AT&T: $0.20 per SMS or $0.30 per MMS, both ways. or text plan for unlimited messages for an additional $20/month
http://www.att.com/esupport/article.jsp?sid=52588&cv=820#fbi...
Verizon: $0.20 per SMS or $0.25 per MMS, both ways. or text plans an additional $5-$20/month, no unlimited option.
http://www.verizonwireless.com/b2c/store/planMessagingOverla...
Sprint: $0.20 per SMS or $0.25 per MMS, both ways. or text plans an additional $5-$20/month, with $20/month being unlimited.
http://www.sprint.com/landings/attachables/
T-Mobile: $0.20 per SMS or MMS, both ways. I think all current plans have unlimited text though, not sure.
http://www.consumerreports.org/cro/aboutus/mission/viewpoint...