Nor from a security one as you'd be mixing passengers screened to two different standards (US and UK).
I dont see how this is an issue - you dont clear security when you arrive in the US - you clear customs. Passengers travelling UK-US already mix, and it isnt a security issue now.
It could however be a customs issue - after passing through immigration and customs, passengers who have gone through usual security as a domestic passenger could easily pass on items that are banned in the US, surely?
Not a security issue, but a possible customs issue.
I assume there is nowhere in the terminal that sells haggis or kinder eggs - two highly dangerous materials that are banned in the US.