If the offending passenger eats a first-class meal, or avails himself of free booze, then he most certainly is taking something (physical) he didn't pay for. Heck, you could extend this to things like using the extra entertainment channels, the hot towels or the extra service from the FAs.

Even if he doesn't eat or drink... if you steal cable TV but never watch it, or if you pirate software but never install it, try arguing in court you're not stealing.

It's very hard for me to see a grey area here.