When I give a song to my friend, its like I put a website in an iframe. The whole site is still there, attributed to the website's owner, the owner just doesn't get money for it.
When I make a design that looks eerily similar to someone else's, I am trying to pass it off as my own. That's plagiarism, and that is the line that is more morally reprehensible because of the implicit deceit.
That's not to say that infringing on someone else's property is not wrong, but infringing with deceit is IMO much worse than infringing without deceit.
Musical works classically fall squarely within the type of protection copyright was designed to afford, the stated goal being to promote creative expression.
Interface designs, though, do not. The problem with claims alleging infringement of designs is that a party may, if courts are not careful, be given what is effectively a monopoly on an idea (Apple tried to get away with this in the early 1980s by claiming that its iconic depictions of common office items were protected by copyright).
In any case, this is not the place to get into legal technicalities but simply to point out (in answer to your question) that the principle of fairness embodied in the copyright laws is to protect original expression (such as is embodied in musical works) while not setting up arbitrary barriers to originality (such as would happen if a someone were allowed to corner exclusive rights on common ideas such as how an interface might be designed).
Thus, the type of case that might be brought against Microsoft is actually a tough one. It can be won but the showing that needs to be made has to be very strong.
I understand that there is a philosophical argument against the very idea of copyright, and the issues are exacerbated in our era of intense inter-connectivity, but the above sets forth the key principles as the existing copyright laws seek to uphold them.
I would imagine that many people believe that the morality of stealing depends on who you're stealing from. Additionally, many probably don't consider it theft if there isn't an immediately apparent victim.
Plagiarism != copyright infringement.
I don't understand or intend to comment on any of the legal aspects of this dispute, but as far as morality goes, Bing travel looks like a clone of Kayak with a different logo, and that doesn't feel fair to me.
Furthermore, Kayak is a clone of Fly.com's design, so Kayak really has no ground to argue about Bing allegedly copying them.