Because it is a more accurate term, just not in the way that these guys are applying it. If you can't accept speed metal as a legitimate name for a genre then you can't accept ANYTHING as a legitimate name for a genre.
Basically, speed metal (as well as power metal) was the last metal genre to remain completely without hardcore influence. Thrash metal was just speed metal infused with thrashcore (a derivative of hardcore punk) and yes, the two often overlap, but they are two distinct genres. Death and thrash overlap, black and thrash overlap, black and death overlap, and so on. That doesn't mean everything is speed metal.