well actually as it was explained to me, this is because visas are checked just prior to security now whereas before at T4 and T1 and every other non US airport with flights to the USA, these checs have been done by a "document expert" at the boarding gate

Check in desks give the opportunity to have passports checked as does the fast bag drop, but OLCI and hand baggage only, these ir no such opportunity.

This is just a rephasing of a process common to every airport and not a distinguishing factor at LHR.

It is for your good as well as the airlines. A colleague of mine has a "refused entry" stamp on his record as his italian passport was not accepted on entry to the USA but wasnt spotted ex CDG