First of all, that's not a discount. The plan costs $30 every 30 days. That's what it costs. For anyone. I don't have to part of any big corporation to get that rate. It's an inexpensive plan, but not a discounted one.
Whenever there is a reduced rate available to some customers (based on where they work, for example), but not to all customers equally, then all other customers who are on a plan without a discount subsidize customers who are on plans with discounts.
I am not opposed to the concept of bulk discounts. Except in this case, there is no guarantee of bulk purchase. As I pointed out before, most of these employers have discount agreements with ALL 4 national carriers, which means there is no added incentive to choose T-Mobile than if they didn't have the discount with any of the carriers. Because of this, this does NOT qualify as a bulk purchase.
At the time these discounts were conceived, not everyone had a cell phone. That key fact has changed. When it started, it was a way of inducing people to get into the mobile market at all. Today, that's no longer the case. With or without a discount, most everyone is going to get a mobile plan. At that point, the only justification for these discounts is bulk purchase, which, as I outlined above, this is not.