They signed an agreement not to that is binding. It was a stupid thing to do, but they did it. It just got extended for another 4 or 8 years.
Look up the 1981 consent decree, or go here: http://www.wnd.com/2012/11/gop-legally-barred-from-fighting-vote-fraud/
The consent decree was just renewed, there was a post here on FR about it in the last few weeks.
It got extended for four years from December 2013:
In deciding the case, which stems from a 2008 lawsuit brought by the DNC, the district court clarified ballot security efforts as “any program aimed at combating voter fraud by preventing potential voters from registering to vote or casting a ballot,” and upheld the consent decree while adding a Dec. 1, 2017, expiration date.
The RNC appealed, saying that its decades-long compliance and good faith should justify ending the consent decree.
The Third Circuit upheld the district court’s decision and modification, saying that just because the RNC had largely complied with the order, the “mere passage of time” does not count as a change of facts that would warrant lifting the order.