Allow me to fill in between the lines:
"What is this, some sort of (mostly still) free country where people are not given harsh punishments for all behavior that prioritizes the individual over the group?"
(sarcasm implied)
"By accepting a modest delay in going straight, the opposing traffic has saved the left-turning traffic waiting an entire light cycle to turn left, as well as saved an equivalent amount of time for all the cars that otherwise would have been stuck behind the left-turning car. In situations where there is so much oncoming, straight traffic that a left turn would not be otherwise possible during any part of the light cycle, the Pittsburgh left can allow a line of left turning traffic to proceed incrementally."
If people just wanted to turn left, this would happen everywhere. Instead, I see it almost exclusively in cities with narrow streets lacking left turn lanes. In extreme cases, it's even personally beneficial for the yielding driver - the alternative is sometimes gridlock that circles back around to them.
None of which makes it safe, but it's pretty obviously common for reasons other than selfishness.
(We don't seem to have this problem out here in California, where every street has at least four lanes!)
Wikipedia does hint at a legal kludge, though - "I judged oncoming traffic to be moving so slowly that I could safely turn". The idea I guess being that it's not a change to driving rules, it's just slow acceleration and a legal turn.
I can't imagine that would hold up real well legally, especially if people are flashing headlights and making an invitation. On the other hand, I've never heard of this leading to a ticket in Boston - it just can't compete with the other violations that happen constantly!
Thank you. I'm going to use this reasoning next time someone complains.