Is your entire trip to/from LAS on AA? Or are you changing from AA to another carrier in DFW? If you're on AA all the way, and there is a delay beyond your control that causes you to miss your connecting flight, of course AA would rebook you without question. If however, you have two separate tickets, one on AA, say from DFW-LAS-DFW, and say another connecting ticket on DL between SAT-DFW-SAT, and these tickets were issued separately on two different airline stock (tickets), then the fare rules for each individual ticket apply. If however, using the same example above, is ticketed all on one airline stock (purchased as one full ticket, and not separately), then again, the airlines would protect you on the next flight, if you missed your original due to a delay beyond your control.
Hope this answers your question.
We are the people of American Airlines. And we know why you fly.