The government is shifting the burden of tax collection onto out-of-state merchants. If the states decide to pay those merchants for the extra time and effort that they will have to spend to perform tax collection, that would be a different story.
The federal bill allows the states to cooperate in the SSUTA, the Fed govt itself isn't collecting this tax, which is why the bill is only 11 pages long. The states want this tax, it is owed to them, they want the Fed govt to allow them to set up a nationwide system to let it be easy for them (and for easy for stores). States can participate, the Feds aren't making it mandatory for the states to do so.