The "ban" is non-binding.
The ban is not so much a rule preventing earmarks as it is a statement saying that earmarks should be stopped. Senators will still be able to use earmarks that they deem beneficial for their constituents. All the "ban" does is give the illusion something more significant has happened when nothing actually changed.
The "ban" would only last 2 years.
If the republicans were seriously opposed to earmarks, why not propose a longer, if not permanent moratorium? Because the two year timetable allows them to use the ban as campaign material for the next election. From there, they can drop the issue if they win control of the Senate or reuse it if they don't.
Republicans do not control the Senate.
While republican lawmakers still do have the power to add in earmarks, their effort is hampered by their minority status. As a result, an earmark "ban" would impact them less than it would with democrats in power.
These very obvious clues illustrate a very obvious tactic: Advocate a very popular, yet toothless policy to game public opinion. Sadly, both a complacent press and democratic inability to produce a counter message has allowed this tactic to succeed.