People are entitled to object to King's use of Wally if they choose, of course. I've already admitted that I'm pretty protective of Superman as a character, myself. My reaction to Heroes in Crisis is my own, and no one else's.And here's the real answer. I suppose you'd feel differently if he did what he did to... I dunno, Kara?
You understand that Wally was the Flash for a generation of fans and had a well-developed history going back decades before that?
Off the top of my head, however, I've seen fans complain about King's portrayal of the following characters: Supergirl, Adam Strange, Alanna, Mr. Terrific, Orion, Lightray, Highfather, Booster Gold, Guy Gardner, Martian Manhunter, Ice, Fire, and probably a few others I'm forgetting. Some of those are, let's face it, barely D-listers. Yet to at least some comics fans, they're all untouchable sacred cows, allowed to be written in only the most narrowly-defined and calcified of fashions.
I saw one reviewer -- a generally smart and perceptive guy, with whom I've had a number of pleasant online interactions -- drop The Human Target less than halfway through its utterly brilliant run because (a) Ice suggested that people respected Martian Manhunter more than they actually liked him, and (b) one panel implied that he and Fire were engaged in a consensually kinky sexual relationship. Those are reasons to abandon an otherwise great book? Really? I mean, certainly it's his prerogative, but it seems kind of petty and provincial to me.
As for Kara specifically, while she doesn't fare as harshly at King's hands as Wally does, there are Supergirl fans who HATE Woman of Tomorrow. The reviewer I mentioned is one, and the guy who runs probably the most Kara-centric blog on the web is another. The latter fellow is actually dismayed that his favorite character is getting a brand-new big-screen adaptation, because it's based on King's book.
I think that's unfortunate, for those fans more than anyone else. The book is wonderful, beautifully written and illustrated, remarkably moving and powerful. Kara's rougher personal edges, and her arc over the course of the story -- which carries her to a dark place, from which she is ultimately redeemed by the power of her own kindness to another -- in no way disrespect her character or compromise her in any meaningful way, IMO. Indeed, her heroism is affirmed in such an emotionally affecting manner that I read the book's climactic sequence through a sheen of tears. I regret that some people -- especially people who are dedicated fans of the character -- can't experience the book as I do, because they demand that Kara be portrayed in only a single limited way, and can accept nothing other.