They say United should have given Dao a written copy of the federal rules regarding how airlines may proceed when flights are overbooked even though the flight was not overbooked.
IDB doesn't care if the problem arose from overbooking or something else. You get the same thing if they have to use a smaller plane, you get the same thing if they can't use a seat for some reason. (And any safety-related issue takes a seat out. For example, the seat I had that decided it wasn't going to lock in the upright position.)
They say they 'generally' pursue enforcement action when there's a pattern or policy of non-compliance. That might be true but it sounds like they're saying they don't do shit when the non-compliance is occasional. They didn't find evidence of racial discrimination which could mean somebody actually investigated the matter, or they didn't find anything because they didn't bother looking.
The only wrongdoing they found was a failure to give him the information--something that would be hard to do as he left. Of course they aren't going to take any action about that, while technically wrong there's a substantial extenuating circumstance. (The purpose of the law is to ensure passengers are informed of their rights and aren't mislead into accepting less than the mandated compensation.)
The airlines have established rules for how IDBs are selected, it would not be hard for the DOT to check the rules vs what happened. (In all probability it's simply a matter of asking the computer for a ranked list of those who get picked, combined with a bit of human judgment as the computer doesn't know all the facts and it may select someone that can't be IDBed.) If that list matches what happened, end of discrimination investigation.
I wouldn't say they're going against the facts so much as simply ignoring them.
You're the one ignoring the facts.
The problem is he went against your faith. You're reacting about how a creationist does to evidence of evolution.
Hint: people posting faith based dogma are the ones who couldn't find a fact if you led them right to it.
And you've been lead to the DOT report.