I get it till I don't.
What I get: In-store activations and upgrades are time consuming in many cases. I think every time I've gone to a TMO store to upgrade a phone for myself or a family member, at least one other person/group is there doing an activation that for reasons I cannot fathom takes at least from when I arrive at the store, wait 30-45 minutes, finally get handled, get activated... and that other person is -still- going. It's a huge time-sink.
What I don't get: charging the full price for online activation, when you're not involving nearly as many employees. It will potentially backfire, I know if I'm paying 35$ whether I do it myself or have someone else do it for me, the value-prop changes and I might just go in and further load their stores.