I have no objections to an annual pass costing the same or less than a single day, particularly because a day ticket is so expensive at our parks. I had a Universal Studios Hollywood pass once because it cost the same as a day ticket. The same year I also had a Six Flags pass that cost marginally more than a day ticket and gave me access to every Six Flags park in the country. It's really not unheard of, and given the inflated day ticket price of our tickets, the actual level of discounting is not drastic.
If a tourist -- or even a local planning a one-off visit -- is buying an annual pass instead of a day ticket, what is the likelihood that they'll use the pass to the point where it negatively impacts the company's balance sheet? Pretty low. There's no fair way of making more money from tourists than locals... again unless they had peak/off-peak passes that meant that tourists in the peak do pay more, just like they do for accommodation, flights, etc.
The figure I'd really like to see is average number of visits per pass. I would guess that despite the many families who look like they're just there to get their money's worth (cheap refill drink bottle in hand, kids that look barely excited to be at a theme park, lunch in a cooler in the car), the average person is using it 2-3 times total and probably dropping $20-30 each visit on food and merch.
As for throwing in the bring-a-friend deal, I suspect they're really banking on getting as many of these people as possible to upgrade to their own while they're there with friends who have the pass. Not to mention it's the lowest period of the year, so giving the gate away is probably not going to have much of an impact or cost to the parks' operations, and it'll surely boost F&B/merch revenue.
I think the economics is there for cheap passes, I just think they're missing a huge opportunity without peak restrictions.