[The Federation is] Entitled to [expand] by who?
The big issue is the Federation's values which they justify themselves by are not universal ones and their causal assumption they are is part of the reason the war is happening. The Federation definitely didn't do anything other than be a target there but I understand the Klingon's logic they believe they'll be beaten economically and culturally if they don't unite.
Why allow them to expand at all?
[The Klingons are] Destroying a threat before it arrives.
It's really hard to tell the extent to which you're just pulling our legs here, playing devil's advocate for the sake of discussion, and the extent to which you actually believe what you're posting. Hopefully it's less of the latter than it seems at face value, because it's kind of disturbing.
Expansion of an alliance through consent based on mutual benefit is, by definition, legitimate. Nobody involved has any reason to object. Expansion of an empire through conquest is, by definition, not legitimate: the party being attacked and conquered has serious grounds to object. The two modes of expansion really aren't analogous.
On a serious level, I do think what's going on is really a fact we're seeing real-time Values Dissonance leading to cultural misunderstanding on a fundamental level. I don't think T'kuvma ever really believed the Federation came in peace and I think they are seeing expanding of the Federation's borders as a genuine attempt to win by hook and by crook their territory for them. The Klingons see them doing as immoral because they're trying to bribe their way out of the territory rather than engage in honest battle.
This, now, this contains a meaningful insight: T'Kuvma and (most of) the Klingons are simply incapable of believing or comprehending the Federation's motivations.
Granted, any society that sees mutual benefit as "bribery" and violent coercion as "honest battle" is a pretty f'd up society, and obviously doesn't deserve the respect of anyone not a member of its self-serving ideological cult. Nevertheless, if that's the viewpoint to which one is acculturated, it's easy to see how it leads to paranoia about powerful outsiders.
Frankly, I've never found the Klingons all that interesting, because they're just so damn implausible. They make convenient villains for story purposes, but realistically the notion that a society organized around glorifying violent conquest, and disdaining peaceful/intellectual pursuits, could ever achieve the tech levels necessary for interstellar colonization, is pretty hard to swallow.
The closest anyone has ever come to making Klingon culture make sense (IMHO) was John M. Ford in
The Final Reflection, but unfortunately most of his insights were rejected during the TNG years, which portrayed Klingons with all the nuanced sophistication of a biker gang in space. To the extent that I hear some echoes of Ford's work in the Klingon scenes in STD, that's welcome, but there's not really enough of it there (at least so far) to make it all seem plausible.
To me the Klingon scenes were the best part of the two pilot episodes. They did a great job of actually making Klingons finally seem like actual warriors with a culture as opposed to the long-haired, leather-wearing orcs with metal boots.
I hope we get to see a lot more of them and with T'kuvma dead I still get the feeling we will be seeing many flashbacks of him. Or perhaps he really isn't dead?
I think at the end of the day, the Klingons work best as an allegory. But whether they're an allegory for the Soviets (as originally conceived), or for feudal Japan, or the Mongol hordes, or (as seems to be the case here) for ISIS/Daesh, is almost beside the point unless they also make sense on their own terms in-story. Unless STD makes them work on that level, the more the Klingon war dominates this season the more tedious it's going to be.