What makes one speed limit more “enforceable” than another?

From the Journal Star:

Nick Stoffer, a traffic design engineer, told members of the traffic commission that the Federal Highway Administration informed the Public Works Department that a blanket 25 mph speed limit “was not enforceable” and should be avoided. Stoffer said the state’s Department of Transportation echoed the administration’s thoughts.

Huh? Why is a blanket 25 mph residential speed limit “not enforceable,” but a 30 mph speed limit is? Wouldn’t one be just as enforceable as the other? Further confusing matters is this:

Instead, city officials will consider requests from neighborhood groups, district council members and others who want their residential streets or subdivisions to have a lowered speed limit.

So, if every neighborhood in Peoria requests a 25 mph speed limit, the city can do it. Apparently under that scenario, it’s enforceable. So what’s the difference between that and changing all the residential speed limits at once?

One more question: Why does the Federal Highway Administration have anything to say about speed limits on residential city streets?