Help the Russians set up a program that allows them to create a GPS system that will compete with the U.S.
or
Help the Russians set up a program that allows them to create a GPS system that will compete with the U.S........and which could be actively shut down / hacked/ sabatoged within U.S. borders if an "incident" ever arose. And which Russian "allies" are likely to sign on to use this alternative? Why China, North Korea, Iran, and Syria of course.
If all the revelations about the NSA show anything, it's that everyone is busy spying on everyone. Therefore the U.S. should presume that these stations will be used, at least tangentially, for that purpose. Note that that is not necessarily a reason to decline the request. If properly managed, it could be used by the U.S. security apparatus to better monitor and determine Russia's own capabilities. It could also be a useful way to "leak" sensitive sounding FUD back to the motherland.