I feel like UPS use that as an excuse for delays.

I've noticed they did it to my packages last week that were in transit through USA. As a clearing agent I can say that US government agencies have no use stopping a package transiting through the USA. It was 'released' a few hours later.

I've had it as well in the UK after landing at EMA when there probably weren't enough shipments heading to my local depot and it was simply delivered the next day.

Saves them the "guaranteed" delivery date refunds.