For starters, RIM does it already. There's a precedent set, and if any carrier was to object, they'd have to eliminate BBM as well unless they want a lawsuit from Apple. So it's either allow Apple's iMessage service AND BBM, or eliminate both, because one can't live with the other not being allowed. Secondly, as already mentioned and known, several apps already allow this, across multiple platforms (iOS, Android, etc.), and while the scale of users may not be as large as RIM's and Apple's, it's still the fact that this messaging service is allowed. Thirdly, other services provided by carriers already have substitutions. Voice calling has Skype, Google's services, etc. And again, though the scale of users may be small as compared to VZ's and AT&T's customer base, it again falls back on the fact that it, too, is allowed. So why couldn't messaging? And fourthly, iMessages isn't "free". It does still use data, though a very small amount. Carriers will receive some benefit from this, with a marginal increase in chances of overage charges.