Just my personal opinion, but think of this as another form of customer segmentation - competitive, low fares for customers who are willing to fly at the times they booked, with a fee charged to customers who decide to change their plans.

Definitely many customers, especially business travelers (many who end up buying the same low fares as leisure travelers), value getting home early and this fee bascially monetizes that value, because a customer booked on a 6pm flight who wants the flexibility to fly back at 3pm if they happen to be ready is getting a differentiated product to a customer who books 6pm and doesn't need the option to fly earlier.

Southwest recognized this with their longstanding policy to require customers to buy up to a full Y fare to fly earlier, they realized many people bought Y fares for flexibility and allowing free standby to everyone would diminish the value of their full fare product.

Now as far as 2Ps and 1Ps not being exempt, there is a silver lining - fewer customers competing for standby, so more of a fighting chance for those paying the fee to actually clear standby instead of waiting at the airport. The beauty of this fee is the choice is put in the hands of the customer, and it makes the low fares that customers value more sustainable.

Possibly there will be kinks that need to be worked out, so definitely continue sending UA your feedback!