The reason it didn't get stopped is because it isn't a DQ offence. After Judah low-blows Floyd & hits him over the back of the head the ref calls time. As soon as this happens, there is no issue with Roger entering the ring, certainly not in terms of it breaking rules. What happens from there saw Roger, Ellerbe, Zab & Yoel Judah all lose their purses for bringing boxing into disrepute. However, it is only a disqualification if the corner gets onto the ring apron & into the ring whilst the fight is in motion. As soon as the ref calls 'time', it is not in violation of the rules. If you don't believe me, look it up

As for the idea that Mayweather was faking

Zab was about to get stopped so threw all the power he had into the low blow & shot to the back of the head. Probably why he couldn't stop Ellerbe...

... which is the real travesty of that night

Oh yeah & Floyd uses the elbow to the extreme, but hey show me a great fighter who didn't use some illegal technique in the ring to mess with his opponent.