I doubt that these senators have considered the possibly that being able to shut down an offending site (say Bing, Google, Hotmail, Yahoo, Youtube, ) wouldn't have significant collateral damage.
This is equivalent to shutting down an entire mall (which happens to include an office for the County Tax Assessor, small FBI field office, post office and police substation) on the account of one bad employee grossly (mis)representing the interests of the merchant renting space in said mall.
Bottom line:
Merely having such a kill switch is not a license use it indiscriminately and not face the consequences of its misuse.
(Notice that engineers are required to retain errors and omissions insurance for bad engineering decisions, but no legislator is required to retain insurance for passing of bad laws.)