I don't see it as an idiom: You can easily diagram it, and the syntax is ordinary. But it's true that the meaning of shame softens when it's used this way, so I'd call it an expression.
Someone above found this:
"a fact or circumstance bringing disgrace or regret: The bankruptcy of the business was a shame. It was a shame you couldn't come with us."
Not quite. Regret, yes. Disgrace, no.
"It's a shame the game got rained out."
"It's a shame you missed the end of the movie."
"It's a shame your guitar string broke."
No disgrace there! It really just means:
- it's too bad.
- it's unfortunate.
- it's regrettable.