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).
I don't see how you are getting here. Are you talking about tagging IFB (Invoke for bonus) or IFE (Invoke for effect)? If you tag a IFB then the tagged shouldn't get a fate point. I feel that a IFE, being narrative control, should have a fate point given to the tagged, but that's just me personally, even though I know that the RAW might disagree with me. I see it as IFE giving narrative control. If someone is taking NC over someone then that person should get something in return. But most likely I would just use a tagged IFE to lead to a compel to give a fate point, which seems to be okay.
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.
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?
I agree with ScottMcG about that ruling. The RAW is does have some funny circuitous wording. The difference I can tell is that according to RAW you can not tag a compel, a tagged IFE would not give a fate point, but a tagged IFE that leads to a GM compel would get the fate point. I know it is a very round about way of doing things, and if you want to limit this and come up with a simpler way of doing things, I think you might even get encouraged to do so seeing that Fate in general is all about Moding for your own purposes. What ever makes it easier for you.
The difference I see is that a player is more likely to try to go overboard with a compel, or at least try to get the most out of it. Where as the GM might want to try to fit it in the story better. I see it as a GM having more control over their npcs. That's one thought anyways. It really comes down to how you campaign and your group runs. You might want to leave these things, like IFE leading to a compel during battle, up to your players.