@Doomer I mean, refusing the king is enough to warrant a beheading in some monarchies. Turning your back on the king and walking away being equivalent to treason is pretty normal for most of them. The royal family, a duke, or republic member might be able to get away with it, depending on the power structure of the monarchy, but- regardless of her contributions and displayed power and connections- she's just a Viscountess. She's the one who wanted to be treated as a citizen and noble of that kingdom, to act out after that decision on her part makes it all the more of a slap to the king's face.
Moreover, this chapter starts by clarifying that a bunch more nobles have come in to the capital, prior to the first meeting. So there are a number of the nobles there who aren't directly familiar with her or her contributions, and who may perhaps not even be aware of who she is. Hence, reacting in such a manner is all the more understandable.
In support of that, only a minority appear to have responded negatively. On the other hand, the majority would have responded in cheering for the returning hero, which is something that'd easily rope society-concious nobles into also participating in the behavior, meaning that the previously negative minority would have had no choice but to switch to cheering at that point [admittedly, the quickness of the transition doesn't make that underlying step entirely intuitive].
That said, yes, rich/societally influential people tend to act like children. Look at any person in the modern era, an era where the inherent self-righteousness of the upper class is significantly decreased [unimaginable as that may sound] compared to that possessed of historical nobility. Of course, you could easily argue that modern freedom of expression and lack of fear of reprecussion makes up for any decline, but the point is, entitlement tends to follow along with high social affluence (or at least, entitlement becomes more readily visible under such circumstances).
In any case, regardless of justifications or premises, a king can't simply remove noble titles without cause unless the royal family is incredibly well-positioned and centralized in their control of power. Otherwise, the nobility gets edgy over the seeming capriciousness of the king, and you end up with a rebellion, a rebellion heavily supported by any deposed nobles. Alternatively, the nobles take their assets and move to a neighboring country, which is also not a good outcome.
Well, the characterizations in this manga aren't all the complex, either, so one could argue this is all just after-the-fact justification for the author's simple approach.