The issue here is the "Protection of Children" clause in the amendment. With the elections approaching quickly, voting against that amendment would be twisted into "Your representative refused to prosecute child porn!!!elevenz!!111!!!" commercials.
No one really wants to spend money trying to explain why that's not true, and with a promised veto from the White House, the Dem leadership probably did a quick count, wrangled enough members into voting no on the final bill to make sure that it would be short of the 2/3rds necessary for an override, and released everyone to vote how they liked (for their own protection) on the amendment, and the folks in the vulnerable seat to vote for the final bill.
Now it gets kicked over to the Senate, which is slightly less vulnerable to this sort of election blackmail. If it doesn't die there, it gets vetoed, and not overridden.
Sound and fury, signifying nothing.