The problem is you all are talking too broad so then everyone is right... and people talk past each other.
'manipulate lines' is too broad. Really there are two separate topics - Attraction Capacity and Posted Attraction Wait times.
What could get Disney in trouble is not 'running lines longer than they need to be' but rather 'false advertising' with regards to waits in an effort to sell line-skips that are unneeded.
Disney is under NO obligation to run attractions as most efficiently as possible. As a broad generic example, If BTMRR can run 6 trains and Disney choses to only run 4.. there is NOTHING anyone can say about it. Heck, they can run 2. The only party that has any say in how Disney runs their attractions for capacity is Disney. There is nothing FTC or anyone else can say about that.. The motivation is only on Disney in terms of managing their customer experience.
And as mentioned before, Disney has 'tolerances' it normally considers acceptable waits - It does not run all attractions at full capacity at all times. This is normal operating procedure. They would scale up when demand exceeds thresholds. So maybe they run 3 trains as a minimum baseline... and will not run more even though they can... unless the avg wait starts exceeding 75mins. Then decide if they will add capacity to get things back under the target.
All of the above is industry norm and happens all the time. So if the wait for BTMRR is higher than it *COULD* be... no one can say squat about it except to complain to Disney as a customer from a experience/satisfaction angle.
Second, there is the topic of advertised wait times. This is an area that we already know is a published time that is not strictly accurate. The advertised time is both an estimate based on current conditions, and patterns. There is also signs that Disney use wait times to influence customer behavior and are not always STRICTLY based on current demand.
These wait times today are provided as a CONVENIENCE to guests - and Disney can defend its lack of accuracy on many fronts. And no one can say much about it.. they offer it as a convenience and people would have a hard time arguing consumers are being harmed by Disney over-estimating waits.
Where things change, and there is a potential argument, is now advertised wait times could be used to sell ticket opens like Genie+/IAS. So the argument is Disney can inflate wait times (either purely in advertised times, or through reduced capacity) as a means to push customers to buy passes.
Let's be clear...
Actual waits being long, even tho they COULD be shorter, and selling passes based on that... No one can say squat about it! There is no regulation that says "you must run rides as hard as you can!". And Six Flags/Cedar Fair would have been crushed ages ago if that were the case
But there would be potentially more scrutiny about how Disney ADVERTISES wait times. Because now we are in the space where one could make the complaint Disney is falsely representing the value/gain their products offer. So if Disney were to say, systematically over estimate their wait times in an effort to make their line-skip products more attractive - This is where there is room for customer complaints and scrutiny from consumer protection laws. It's not too far from an example of selling a 5lb bag of potatoes that really is only 3lbs - it's misrepresentation.
But Disney's defense would be to argue the poor estimations come from other legitimate reasons, not from motivations to defraud customers. Maybe ultimately after some regulator scrutiny there would be some codified expectation of what wait times estimates should be, but there is a ton of room for Disney here to justify their current behavior. Monetizing line-skips increases scrutiny, but the behaviors that are in play today are not suddenly taboo. Disney would just have to be able to defend their estimates to show they aren't doing it to intentionally manipulate buying patterns.
So the only legitimate area of concern is how wait times are shown vs actual wait times... NOT that actual wait times are long. No one can force Disney (or another operator) to run attractions at higher capacities even if they sell line skips.