Integration issues are always the other guy's problem. Exchange's calendaring does suffer from some legacy issues, but I'll wager the Schedule + format they're using pre-dates the iCal "open source" standard. But it's Microsoft's fault they're not compatible.
Open source and open standards are two completely different things.
But it's Microsoft's fault they're not compatible.
That's pretty much the way Microsoft works. Microsoft just finished ramming OOXML through the ISO using every dirty trick possible to head off the possibility of a truly open format becoming the office document standard.