Apparently it's part of the standard rules for secondary and tertiary attacks granted by powers.  I guess the idea is, if you wouldn't target an ally with a single target attack power to begin with, there's no logic behind you targeting an ally with single target attacks granted by the power.

Now area attacks, on the other hand, are a whole different story.

I can agree that it seems in flavor for a Wild Mage to occasionally be a threat to his own party, but we've been having issues regarding 'friendly fire' lately as it is, so there's no real reason to disregard the rules on powers.

A lot of tables have a 'friendly fire' rule that simply doesn't allow you to target another player without their permission.  I propose we adopt this strategy.  No matter how tactically sound it may be to blast enemies adjacent to another player, I don't think anyone sane signs up to get regularly pummeled by their own allies.

Or, put another way, “Friendly fire isn't.”.

The way I see it, if you wouldn't want your Ranger Twin Striking you, you probably wouldn't want a Sorcerer including you in their Blazing Starfall!

This does seem to place an additional burden on those characters with Area powers to seek out ways to lessen the impact their powers will have on the party, but I think the problem here is that we focus on creating our own characters, and do not place enough emphasis on team tactics. 

For example, if you like having AoE damage in your party, then you should consider making your nuker's job easier by taking forced movement powers so that you can herd enemies into position- as well as features that let you move away from/out of the blast.  These things exist, but nobody takes them, because of the opportunity cost- a feat taken to evade a friendly blast is not a feat taken to advance your build, after all.

Since we are a team, we should think about ways to act as a team.

Short and Sweet: if you're the kind of person who thinks throwing a grenade at a friend is ok if you can take out some bad guys, nobody wants you on their team.