Simple: the Dems (Feinswine and Upchuck Schumer among them) were hot to stop this bill. The AWB and gunshow amendments were intended to do 2 things: 1) make the bill unpalatable to the more conservative Pubbies and 2) be used as a club to smack Pubbies around with, since they would be on record against a ban on "evil cop-killing assault weapons."
Fortunately, IMHO, the Dems didn't really get what they wanted. Instead, they are on record (especially Kerry and Edwards, who flew back to DC just to vote to take away people's RKBA) as being against the rights of gun owners. We had Fat Ted (Drunken Murderer, MA) foaming at the mouth about banning the ammo from 30-30's, because they are "armor-piercing, cop killing bullets" (GMAFB!!!). No, the Dems got nothing. So, lawsuit protection will be delayed for another year or so, until the Senate Dems officially commit suicide in the South. None of the lawsuits has yet born fruit, nor will they (to allow such a suit to succeed is to guarantee that some lawyer - plus 10,000 of his buddies - will file another suit the next day against GM or Ford for the actions of a drunken driver - and THAT would destroy commerce in this nation). The only downside is that the costs of defending against these suits will continue to accrue until a clean bill is passed in the future.
Meanwhile, Feinswine and Upchuck will have to find another way to sneak the AWB renewal/expansion through...as the clock keeps ticking. Can you see John Effing Kerry flying back to DC to vote in August or September to - once again - limit the rights of law-abiding Americans to purchase firearms? It'll play well in the media, but the same folks that voted against Gore in 2000 on the basis of his stance on guns will do it again.