I'm guessing there must have been some structural reason they didn't include it, but that's just a guess.
In any case, though, I very rarely used it. Even locked up, I never felt good about leaving my MBP unattended, even on a good tether. They can be cut without too much difficulty and can, apparently, be picked with nothing more complicated than the cap of a cheap Bic pen. Plus, leaving your machine unattended leaves it exposed to some clumsy oaf coming along and knocking it off the desk, or getting curious and looking at it and then spilling their coffee all over it, or deciding that maybe they can yank it off the cable, or the like. I would never leave my precious MBP unattended, even tethered! With the newer models so much thinner and lighter, picking it up and carrying it with you isn't such a hardship.