In actuallity, the Boy Scouts does little to "carefully screen their leaders". It would be OK with me if they did, but how do you identify a "known homosexual"? The scout leadership application form only askes if you have a condition that would disqualify you from actively living up to the Scout Oath and Law? Conditions such as being an escaped felon come to mind, but the question is vague. There is no specific question about homosexuality or belief in G-d.
The best example I know is a troop in the South San Jose region in California that once activly advertised that their policy would be different from the national organization in that they would not disqualify a homosexual from being a leader in their troop. (They felt the national leadership was sending the wrong message.) Their charter was not pulled as they had done nothing specifically to act on their pledge. Finally a young man from San Francisco, who was an openly homosexual man, volunteered to drive the 35 some miles each week to their meeting place and applied to be an assistant scoutmaster. The troop denied his request on the grounds that they had no need for additional assistant scoutmasters, and that was the last I heard of the event.
The BSA does not screen for homosexuals. What they screen for is your legal records. While all screening used to be left up to the sponsoring organizations, the sponsors did not always step up to their obligations. The BSA now takes your Social Security number when you register and has a security organization check out your arrest and conviction records nation wide. If you have a felony record, the local Council will be notified. If you have a conviction involving sex or children, you will be rejected outright. I'm not quite sure how other convictions are handled. If you are rejected, you will have the opportunity to examine and contest the information.