For me I look at it from the series point of view. During the escalation to the final confrontation Harry is constantly being beat on (and starts gain fate points from the all the consequences and compels he's getting) and then goes uber and wins (dumping a lot of the faint points he just gained to win). The gaining a fate point even if tagged, for me anyways, is more in sync with that. Plus it would depend on the group. I don't believe my group would abuse it like you think.
I mean that players will naturally do things to avoid giving the enemy an advantage. Making tagged Invokes give a Fate Point like Tagged Compels will mean players will avoid doing tagged invokes. You've made those tagged invokes more expensive to do...it's simple economics. So I think the net result is it will be that it makes the game LESS fun, because there will be a lot fewer aspects placed on enemies. It's not about abuse..placing Aspects on allies isn't abuse. This just makes aspects on allies a lot cheaper, hence encouraging their placement over aspects on enemies (which is already discouraged by the more significant resistance an enemy can potentially give).
You'll still have lots of stuff happening at the end as people spend fate points they've accumulated, use tags, and invoke aspects that have already been tagged.
Compels that come from an Invoke for Effect are different, of course, since a Compel immediately puts the opponent at a non-trivial disadvantage. That certainly is deserving of a Fate Point as per normal compel rules.
Yes and No. It is partially semantics, like you say, but it would limit things to a certain degree too. A compel coming straight from the PC could have a much wider ranges of effects, or be much harsher. The compel from the GM would IMO be in a narrower frame of things seeing as the GM would only do the compel if they see it as appropriate and apply a potentially more subtle effect. Plus, as I see it, it also effects if the effected party will get the fate point. If your going with "no fate paint spent, none gained" then the effected party would not get the point from the Invoke for effect, but since the GM initiated the compel the target would get one from the compel.
I mean no offense, and correct me if I'm wrong but it seems to me that you're coming from the perspective that this is a big loophole and could be exploited. I see it as what would be a better story, and what would everyone enjoy better. If that means that A tag could be essentially be used as a compel, I'd go with it. But I still retain all my rights to say to a player "Okay, now your abusing it."
As for this conversation overall regarding how to word things about tagging, I am coming from a purely pedantic standpoint. I am just trying to figure out the clearest way to word all of this (because the book certainly doesn't do it). Is it best to have Invoke For Effect have a nested Compel as an option, or just to explicitly state that one can Compel with a tag? Generally I think avoided nested stuff is less complicated, and mathematically speaking this is certainly so if you would just have 3 options (long story).
Beyond that, I don't see how the player doing the compel without needing a Fate Point is different from the GM here. What can be compelled in either case is very dependent on the Aspect in question. I don't see how an Invoke For Effect limits the nature of possible compels at all...anything that can be compelled is arguably something that can be Invoked For Effect (which would cause the compel), as best I see it. Is there something I am missing here?