From the stories I tell people all the time files*:
My friend, we'll call her N, is a knitter. She says that knitting while taking public transportation is helpful since you have tools on your person should you need to strangle or stab anyone. Now, lest you get the wrong idea, N is a lovely person and generally does not need to strangle or stab people, on metro or anywhere, but certainly we can all appreciate that tools with multiple purposes are good.
Now one day N was knitting, and her seatmate was male and he felt the need to sit with his legs spread particularly wide. As a result, he was encroaching on N's seat and and person, so she pulled out her handy tape measure and offered to measure for him to see if he really required so much space when sitting.
So, imagine my joy, to discover over at the Crazy Aunt Purl blog that this issue has been named - imaginary package syndrome or IPS: "the guys who have an imaginary package so large they have to spread their legs really wide and encroach into your personal space to accommodate that enormously huge imaginary schlong".
Oh, and the guy sitting next to N, he closed his legs.
*So, sorry, for those of you have heard this many, many times.