I haven't noticed that here in UK or in Asia, I take it this practice is common in the US. IMO, it sounds silly, if your flight from A to B happens to stop at C and the latter flt is operated with a different aircraft, it should have a different flt number for route B to C than A to C. It's only acceptable practice imo if the entire flight is operated by the same aircraft and is listed as having a stop-over.

But in the airline industry "direct flights" and "non-stop" means two different things, "direct flights" can have a stopover whilst "non-stop flight" is quite literally non-stop.

I suppose one can assume, these carriers who have these "direct flights" have decided to include 'aircraft change' alongside with its primary defination of disembarking/picking up additional pax at the stopover and/or refuelling.