It seems to me that to make your argument valid you have to be implicitly claiming the following: we're so confident that no fragment of the current Islamist terrorism movement will target subways that we don't need to put in place the security measures that would be necessary if we thought they might; and yet we are so clueless as to when, where and how they plan to attack airplanes that we have to strip-search every 95-year-old/5-year-old/this week's TSA victim stereotype across the entire system.
Besides, you just can't kill that many people on the subway. A single bomb on a 747 can kill 400 people... and with a single gun you could kill thousands. (A knife probably isn't good enough any more since folks will fight back, but with an assault rifle you can probably kill anyone who tries.)