Oh they do train to the rules, but western pleasure is often judged based on AQHA rules at open shows. The biggest reason is that the USEF refuses to make any distinction between western and english pleasure horses, and their events are so heavy on the english, a lot of the WP people just go back to events that use the AQHA rules. Even my 4-H circuit uses AQHA rules for the western divisions... You may be right, but it all depends on which rulebook you quote.
Anyways, the AQHA have done a lot to turn around the image of Western Pleasure that many people still hold as a result of the "peanut roller" days. Nonetheless, headsets are more widely accepted within the "level topline" margin, whereas the USEF nazi's require poll to withers. Most good aqha pleasure horses will be between poll and eartips, leaning more to the eartips for a jog and poll for the lope.
Still, you are correct. Spur stopping is a good way to keep your horse on a draped rein, and if you watch world championships, you will notice a lot of draped reins. Now, really, those horses didn't get to the world show for nothing... if they were really breaking rules like crazy, they wouldn't be competing at that level. It sure as hell isn't an open show.
You can also read earlier posts to hear other reasons why people use spur stopping over bit pressure if you like. I'm not going to reiterate.