Because it's so hypocritical. As I pointed out, people will bend over backwards to ignore the inconsistencies in their own arguments. It's okay for me to tell your character how to act and believe, because magic. I can force you to take extra actions, heal, hit better, take less damage, and a multitude of other things, despite the fact that it makes little sense in the game world, and it's okay because its magic. The character that hates music still gets inspired by the bard. The character that's diametrically opposed to your beliefs still gains HP. And, note, the argument is that people don't want other players telling them what to do. So, when that wizard hits you with Haste and grants you extra actions, he's telling you what to do. But, that's okay because... altogether now ... magic.
Like I said, if the argument was at all consistent, then I'd be a lot more sympathetic. But, it's not. A cleric with an at-will cantrip that granted an ally an attack when cast would pass muster without a comment, regardless of the alignment or beliefs of the ally. Heck, as it stands, clerics (and druids) already have a cantrip that grants buffs on any skill checks. Doesn't matter what your deity is or what your deity is about, drop a Guidance spell and someone gets a +d4 on a skill. My Cleric of War can buff your History check for some reason... oh right.... maaaagic. But, apparently, a warlord doing exactly the same thing but without casting a spell is completely unbelievable.
So, yes, this is why I sneer at the arguments being dropped here. They're about as consistent as a wet cardboard box. And hold as much water. For some reason, we don't see constant, daily agitation to change clerics, druids, battle masters, bards and Mastermind thieves. They don't apparently rate high enough as an issue to get their own forum ghetto, exiled out of the mainstream conversation by constant, never ending, thread crapping by people who have made it their mission in life to prevent any 4e elements from polluting the wonderment of 5e.
If a Wizard casts Haste on you he doesn't force you to make an extra attack. If a Warlord inspires you to take an extra attack he doesn't actually force you to take the attack. Neither are forcing the other character to do anything.
Now go read that above again and let it sink in. Understand?
Okay, now, the Wizard uses magic to enhance your physical abilities beyond what is normally possible. A Warlord requires the target character to be inspired by his words so that he can perform better than he normally would without the encouragement.
Do you see the difference?