The good news is that bags are almost always reconciled with their owners.
I dare say that someone did a calculation and reckoned that the cost of delaying several aircraft was greater than the cost of delaying bags. Think of the issues if transfer bags didn't make it at LHR as a result.
It would have been necessary to put in bag on the same flight as its owner which would in turn would have meant relabelling each bag which couldn't be done until the poassenger had been rebooked. Presumably an unaccompagnied bag has to go through additional screening.
I volunteered to be offloaded by Delta sometime back on a Seattle to Atlanta sector a few years ago for $400 in Delta dollars. There is no domestic baggage reconciliation in the US so the bags went to ATL. I was travelling onto LGW from ATL the following day so remained airside when I finally made it the folowing day but stopped off at the DL service desk to ensure that they were to be put on the flight which I was assured was the case. No baggage on arrival. One bag arrived the following day and the other the day after. I live in West London so that was two courier trips from Gatwick. Can't have been cheap!
Makes you wonder why positive bag reconciliation is necesary on interrnational routes but not on US domestic services. I would be surprised if US hub & spoke operations could survive in their existing form it it were introduced though. I actually doubt that it serves much purpose in this age of suicide bombers. Better get your screening right though.