You don't understand what is happening. UPS made this mistake once and is not going to do it again.

For business customers, accepting a package and failing to deliver for the holidays is WORSE than accepting (a la USPS) with no limits and then having it get stuck.

UPS did this to amazon once, and that was enough for Amazon to decide to build out its own delivery network.

Shippers can do all sorts of things to help make things efficient if UPS communicates the limits. Ie, no sales with guaranteed 2 day shipping 2 days before xmass.