Jim Joyce didn't think the legs were irrelevant.
Question: "Jim, could you walk us through what happened?"
Jim Joyce: "When he [Craig] tried to advance to home plate, the feet were up in the air and he tripped over Middlebrooks and I instinctively called 'obstruction'".
Really, many MLB umpires couldn't pass a simple softball rules test even with an allowance for MLB differences. This is why they have a crew chief.
Then you stick a microphone in someone's face asking the same stupid question 20 times and expect a different answer. Unfortunately, some people feel compelled to provide unnecessary justification to a call.
As a softball player you probably see this a couple times a month, and often to justify a bad call, so why do you think it is different at this level? As is routinely pointed out, these guys are human like everyone else. Like in all sports, the official is the best available, not necessarily the best there is.
Remember when Rich Garcia kicked a spectator interference call in the 1996 ACLS? They stuck a microphone in his face and beat him up. He eventually offered, "yeah, maybe I should have just awarded him a double". WHAT!?!? The call should have been OUT, not cut the HR call in half. We didn't hear much from Mr. Garcia (who was a very good umpire) after that.
Some people just feel if they tell the press something, it will satisfy the morons and they will just go away. Amazing how that doesn't work yet people continue to do it.
All that aside, the legs are irrelevant to the rule. Again, just something people are pointing to as a "justification", but in reality, it has nothing to do with the rule.