I agree that speculation doesn't do much to help us but it's hard not to not think about lol.
Putting people on the HPWL at this point instead of offering outright acceptances does 2 things - 1. Lets them see where else you are holding acceptances and 2. Lets them gauge how interested you are in AECOM at this point.
Since they've already filled the class once, they now want to accept students who will come this fall. If they put a great applicant on the HPWL and they see that person is holding offers at schools that historically they've lost out to, then they might not bother pulling them from the WL unless the applicant writes a strong LOI.
Along the same lines, since the HPWL email asks you about your interest in attending AECOM, it may weed out students who may have wanted to just see if they would get in and would have stayed on until they had to do extra work to still be considered. Both help achieve the final goal of finding capable students (which there are plenty to pick from) who really want to attend AECOM or at least have a sincere interest in the school at this point in the application cycle.
If someone is offended or feels insulted that AECOM pulled them from the WL and decides not to go because of that, then there's a decent chance that they wouldn't have picked AECOM anyways. It sounds somewhat absurd, but I've heard of some stories like that.
Of course, I could be totally off too lol