I think, we have to consider the context here. Ito being hurt/disappointed by a "real" person (a classmate, she looked up to / admired) would indeed be different than her later finding an idol to admire.
Idols in Japan are regarded very differently than the stars in the western culture (whilst they have that kind of stars as well).
An Idol is more of a fiction, a literal ideal (and a form of escapism). These people have to represent that ideal - and either keep their private lives very private or manage live up to these ideals in their day-to-day life. If they mess up in private, that deeply reflects on their perception as an idol - they easily fall from grace. Much much more so and for much much minor things than in western culture.
As some noted, some pop-idols (esp. group-members under the age of 21) are not allowed to have a romantic (or sexual) relationships - by contract. Because it's their job(!) to appear available to every fan('s fantasy). Like getting caught smoking or underage-drinking, any breaks from the "ideal" can result in these idols losing their jobs. If they are lucky, they just have to apologize publically for their bad judgement and sincerely promise to do better, but even then their image is tainted and their career suffers badly from it. (Aside: If Britney Spears was Japanese, her entire career would have plain and simple ended in 2007. Miley Cyrus would have long be a goner as well...)
Other than an actor playing a role, an idol can't ever drop the persona - it's 24/7-acting, occassionally with some acting jobs on top of that.
I don't think we have anything even close to something like that in the western cultures. What may get closest may be personal feelings one may have for a charming / relatable character on a TV show or movie and then hearing the actor/actress act like a bigotted a-h*le in an interview - which could in turn blemish ones enjoinment of the show/movies' character. But, unless they did commit a serious crime, there are hardly any career disrupting consequences for the actors or the show/character in the western world.
Japanese are in some regards even better than westerners in not mixing fiction with reality - at least when it comes to general media-consumption. But they also have this concept of "idols" or "untouchables". This may or may not trace back to Geisha or to some degree the worshipping of the Tennou.
Back to Ito, that means, up until meeting Sumika personally, she was on the safe-side admiring her, as by an unspoken law, idol-Sumika could not disappoint her.
This also may explain her so called "entitlement" - the real Sumika did not live up to the presented ideal. It's far more than just a mental image that Ito alone had, that got disrupted - it's a concept, an escapism of its own, that was thrown into dissonance for Ito. Especially since that ...basically "piece of art"... helped her overcome bad times and some self-consciouness issues; learning that the "artist" was ego-centric, spoiled and ignorant, threatened to turn all her hopes and dreams related to idol-Sumika into straight out lies, as they were now built on the same. ((Even if she to a degree knew, but wilfully ignored, that e.g. idol-Sumika's Q&A answers were written by editors. Because the desire to believe in the dream/ideal is very real. And ignorance is bliss. That's not necessarrily gullibe - that's actually rather human.)) She tried hard to separate her encounter with real-Sumika from her public image. Hence, her outburst when Sumika kept deepening that rift - so Ito could no longer "unsee" the real Sumika.
Lucky for her, and us yuri-fans, it seems the real Sumika is not a lost cause / bad person after-all (albeit much more flawed than her persona) and has some charm-points of her own to offer and for Ito to "admire"...
...So far I really enjoy this story ^^
