I'm pretty sure they'd get a lot more complaints if it worked the way you all want. Push from iPhone happens when you are using iPhone therefore using iPhone battery is appropriate.
Push from Mac to iPhone uses power on the iPhone when you are NOT using the iPhone. Therefore it should be more conservative.
I can (and often do) make hundreds of changes on my Mac in a period of 15 minutes. I sure don't want each one separately pushed to my iPhone.
Also, if I am sitting and using my Mac, I really don't care if my iPhone is up to date at that moment. I care if my iPhone is up to date after I stop using my Mac.
So if it doesn't sync before sleep or shutdown, then THAT is a problem and needs to be fixed. Other than that, this seems like the correct solution.
There may need to be an exception for pushing changes to my iCal onto someone else's iPhone.
MobileMe isn't all about the iPhone - I think that is a little narrow-minded. I don't yet have an iPhone but I know I want all my mac and PC's constantly in sync - don't forget about all the other devices that don't use batteries that would appreciate having iCal info pushed to them.
People wouldn't complain as much if they got what they expected/were promised.
If something doesn't work the way you want but it's what they promised to deliver then it's feedback. If something doesn't work the way you want yet they claimed it would, then it's a complaint.