My understanding, having looked into this before in a retail context, is that legal tender rules hinge on the term "debt". If you buy something from Walmart, you are not incurring a debt, because you do not take ownership of the items until after the money changes hands. Walmart can thus require or disallow any form of payment they choose, because if you disagree, the items still belong to Walmart and everyone is square. It's like vending machines that only accept coins; if all you have is bills or credit cards, you just don't get a candy bar. If you go to a typical restaurant, though, and pay the check after you dine, you
have incurred a debt because the food is in your belly, so the only way you and the restaurant can be square is by you paying up.
If you allow your customers to incur a debt, you are obliged to accept cash, so long as you do not specify that cash will not be accepted
before the debt is incurred. So if OTA puts up a yellow sign before the ramp that says "PIKEPASS OR PAY-BY-PLATE ONLY" (which as far as I know every toll operator does), it's perfectly fine. Cash is only required to be accepted if someone doesn't outright state ahead of time what form of payment you will accept and then allows you to incur a debt.
How are they going to convert the Turner and Will Rogers Turnpikes, which use a hybrid ticket-based/point tolling system for non-Pikepass drivers, in the form of receipts and refunds? Similar to the issue with any ticket-based tolling system, this system would need to record every driver's entry point in order to issue the refunds accordingly where applicable. The article doesn't explain anything about how that will work without paper receipts.
The way that the Turner and Will Rogers work is as follows: say you are going from OKC to Tulsa on the eastbound Turner. The entry plaza for all eastbound traffic is assumed to be Oklahoma City. If you leave the turnpike at any of the exits on the west half of the turnpike, you pay the toll according to an entry at OKC, unless you have an entry receipt showing that you got on somewhere east of that and thus are entitled to a lower toll rate.
When you reach the halfway point, there is a barrier toll, where everyone pays the full amount as if they were exiting at Tulsa. By default, the entry plaza is still assumed to be OKC unless you have an entry receipt. If you are going to Tulsa, now, you are fully paid up and don't have to deal with another toll at all. However, if you exit early, between the barrier toll and Tulsa, you give them your receipt from the barrier toll, and the exit toll plaza
pays you a refund to make up for the extra money you paid back at the barrier toll for the portion of toll road you're not using.
There's no reason you couldn't convert this process to be all electronic, having an eastbound vehicle with no entry plaza recorded be assumed by the system to have entered at Oklahoma City, and calculate all the debits and refunds accordingly. However, I do think that I've seen Pikepass readers already installed at the turnpike termini, so it's likely the system already is recording that data and doing a more straightforward version of the accounting.