That's not always true. My brother has had two DUIs, one in '96 and again in '02. The last one cost him his job driving a cement mixer and landed him in jail for a month. He regained his CDL and today drives one of those monster-huge dump trucks in a moly mine in Colorado.
Apparently, his qualifications and experience trumped his DUI convictions.
This being said, I believe he was the exception to the rule and I remind him regularly how damn lucky he was to land that job and he'd better not screw it up!
He is very lucky and I have heard of a few others bouncing back after DUIs and worse.