Two things. First, the reality is that at rollercoaster speeds, outdoors, with masks on, I'm not really concerned so much about "going through the same air" when it comes to coaster seating. The chances of spread in that context is fairly low. For me it's about two other questions:
1) What would happen if the ride would break down, for example, and I'm stuck in close proximity to these people for an extended period?
2) Is loading full trains (I honestly doubt they ever HAD a full train last night, but maybe over the weekend) worth the anxiety it creates to know that you're more physically close to someone—when loading, when on the ride, when departing—than messaging tells us we're supposed to be?
So much of what the park is doing is so-called "hygiene theater," working to convince us that we're safe by constantly bombarding us with announcements, signage, sanitizer stations, etc. The reality is that you can't walk 50 feet in the park and not see some reminder of the fact that physical distancing is important, and so it's then deeply confusing and anxiety-inducing to have a rollercoaster break those principles suddenly. It's like the station house of Loch Ness is a lawless, rogue hamlet, governed by an entirely different understanding of the virus than 250 feet away where they're keeping you six feet away from each other to watch the Clydesdales.
There's two solutions to this. Either load every other row and be consistent with the rest of the park, or post signage in front of the ride indicating the science that says they can load every row. But given the walk-on nature of the ride, I don't see why they would be prioritizing capacity when it risks puncturing the bubble they're trying to build for guests.