It's possible the impression was the amendment was too far reaching, it did include some considerably lessor crimes.
The Cornyn Amendment is available on-line for all to review, even our glorious senators who are too busy to actually read what they are supposed to vote on. It is not far reaching at all. It basically only excludes those in categories 7 and 8.
The few of them who offered excuses as to why they would vote down such a sensible amendment offered such banal trivialities as "It was only a delaying tactic by the minority party to sidetrack important legislation." Doesn't that sound awfully similar to the excuses offered when they successfully ramrodded ObamaCare on us?