If there's a US airfield within a few miles (53) from the location of the conference, why would the USAF park one of their jets on a civvy airfield?
I presume by CWL you mean Cardiff rather than Cranwell, but a civvy airfield is a lot harder to secure than a military one. Given the NATO summit might be expected to bring forth agitators (and has done so), security would be a fairly high priority thing. BZZ would have sufficed, but I suspect the Secret Service would rather run the show themselves on their own airfield than be subject to MOD Plod, civvy plod and even the RAF Police all having a pissing contest.
A few civvy plod guarding the outside of a sparse airfield seems reasonable, given that the UK has the privilege of hosting the summit.