Because they probably don't really know what they're talking about. Exhibit A: Rep.
Todd Akin, R-Missouri, actually uttered this bit of nonsense last week while defending Bush's troop surge in Iraq:
"Could you picture
Davy Crockett at the Alamo looking at his
BlackBerry getting a message from Congress? 'Davy Crockett, we support you. The only thing is we are not going to send any troops.' I’m sure that would really be impressive to Davy Crockett" (video
here).
Aside from the BlackBerry part – too easy to hit the fish in that barrel – Akin's reference makes him look like an idiot because 1) Texas didn't join the United States until 1845, nine years after the battle, so Congress wasn't in much of a position to do anything about it; and 2) Akin's quote is pretty close to what actually happened: The Alamo commander, Col. William Travis (not Crockett), sent a letter to Gen. Sam Houston asking for reinforcements, and Houston denied the request because his army was not yet strong enough to send aid.
I'm guessing if Akin's historical knowledge is that poor, he probably doesn't know the rest of the story, so for his benefit: The Texans (actually, "Texians") lost the battle, but Sam Houston's judgment to continue building his troops later led to victory over the Mexican Army.