Why "can't" you compare apples and oranges?

[indecizo]

I always read things like you can't compare some things because they are "completely different". I was gonna make an example but I found this perfect:
http://answers.yahoo.com/question/index?qid=20071213185033AA44miz
Look at all the responses completely missing the point.

You can definitely compare apples to oranges. Example:

Obviously this applies to everything, not just apples and oranges and I'm talking about when people say you can't compare things. Also, I don't think there such thing as an unfair comparison.

 

[JeffTD]

You're just looking for an argument, aren't you?

 

[JHA]

Who said you can't compare them? Though the original phrase is invalid in the situations it is usually used.

 

[SocialNumb]

You're just looking for an argument, aren't you?

This