I guess the real reason the definitions are a bit different in Texas is that BBQ and smoking are generally considered 2 different things. While I will smoke a pork butt - many people will just throw it in a crock pot with sauce. Since the sauce becomes the dominant flavor most of the time - it really does not change the finally product that much. When I do smoke a butt, I rarely will cook it over 225. Now with a brisket, or even more so Chicken, it is not all that uncommon to cook it up to 350. While I personally do Brisket low and slow as well (smoke it), many of the more famous Texas BBQ joints will do turbo brisket, or start brisket at 350 and gradually move it down the pit (as additional briskets are put on) lowering the temperature throughout the cook. They can do a brisket in under 4hr. Pretty sure Salt Lick and at least 1 or 2 of the Lochart joints do a turbo method.
I am not that critical to split brisket between smoked and BBQ'ed, but I do suspect the history of turbo-Q might be part of why most of Texas will consider a butt smoked rather than BBQ'ed - and also why butt is rarely sold at BBQ joints in Texas (even when they do sell pork rips and pork steak).