The BOV's open any time the engine has enough vacuum to overcome the BOV spring pressure with vacuum from the engine combined with boost pressure in the pipes. The BOV does not require having the throttle plates all the way closed. The blow off valves can easily open letting the throttle back from full throttle to 50%. Most people do not drive the car at wide open and idle only. The MAF reads air all the time, the ECU does shut fuel off momentarily when you first let off the throttle all the way when the TPS has idle contact, and your above 2500 RPM, at 2500RPM the amount of air moving through the system is still significant, the ECU fuel cut turns off and gives fuel based on what the MAF voltage is for the air moving through the pipes. The stock system dealt with this by recirculating the air so that no new air entered the system, BOV's change this and the ECU can not compensate for it. Tuning many cars with both recirc and BOV shows you lots of issues the BOV cause, removing the BOV and reinstalling recirc's cures the issues. I could talk for days on this subject and tell you about hundreds of cars that had BOV's with issues for years that now have recirc's and run perfectly.
