I wouldn't call this sad in the slightest. It makes the job of a TO significantly easier knowing whether they have to prepare for 20, 40, 70, or 100 players at their event. If the TO's job is easier, your tournament experience as a player will be much better.
Some League Cups also have attendance caps. Preregistration makes this acceptable, as you won't have to drive hours to a League Cup to find out that there isn't any room for you. I'll be driving 3.5 hours to Tennessee in a few weeks to a League Cup that I had to call and preregister for. If I drove all that way to find out that I couldn't play, I'd be reasonably upset. Now, I know I'm safe to show up and play.
As a side note, I'm incredibly happy with how League Cups are shaping up. I have 2 within 30 minutes of my apartment, as well as another 4 that I can and might attend within 3.5 hours. I'm lucky enough to have enough players in my local area who want to travel as much as I do so I'll have a full car for those long trips to split gas money with. These League Cups seem to be roughly as plentiful and spread out as Cities used to be and more stores should be getting the opportunity to schedule them in the coming months.