Bootloader locks are getting more and more difficult to crack as time goes on. Every time the community has success with an unlock, the manufacturers learn from their mistakes.
The Verizon S3 started out with a critical flaw that is NOT present here - the bootloader only checked the recovery partition when flashing, not when booting. Steve has confirmed that recovery is checked on boot (just like it was in the VZW Note2). The final GS3 unlock depended on a leak from a disgruntled employee.
The Note2 unlock was significantly more difficult - all of the vulnerabilities abused on the S3 were closed. We got lucky in that Samsung still made a critical mistake - they used the same hardware root key in all Exynos 4412 devices, and left some security flaws in the bootloader itself.
See Dan Rosenberg's post about the fact that the last round of Motorola unlocks is likely to be the last - with every iteration, the locks get more difficult.
Samsung is (fortunately for us) inexperienced with bootloader locking, but after an alpha test (GS3) and a beta test (VZW Note2), they've learned a lot from their failures.+Adam Overmiller
The problem with your argument is, as Steve indicated, the part of the phone that communicates with the network is the baseband processor, which has always been locked down and not something we care about. Despite their claims, there's nothing you can do on the applications processor side to interfere significantly with the network. About the worst you can do is use excessive data - but this is metered and AT&T makes money off of it! Actually, knowing AT&T's history, they want users to have their devices using excessive background data due to badly written apps that eat it. (See the initial release of the SGH-I777 - the AP Mobile widget would eat hundreds of megabytes per day for some people unless you rooted and nuked it from orbit.)
Your subsidy argument falls apart too - since even unsubsidized phones purchased at full price are equally locked.