Is playing MW2 via AlterIW legal?

Long ago, I bought a copy of MW2. As I run it, it requests me to make a Steam account and assign the game to it — otherwise, it doesn't let me do anything at all. Not even single player. :(

On Google, I searched for "MW2 no Steam". Google came up with 2 things: Some illegal "no Steam" crack and "AlterIW".

I'm almost sure that applying that crack is illegal, but what about playing through AlterIW?

Here is AlterIW.net.

I read some article about it, and what it said was: "AlterIW net is an alternative to steam which also has much servers..."

What do you think about it? Legal/illegal?

What makes things even more complicated is that I'm playing this on a Mac - with Wine.

This is what the lead developer has to say on the subject:

None of the basic game files are modified, most of them are replaced with custom versions or specific versions shipped by IW in patches. Also, reverse-engineering claims are invalid in Europe to my knowledge, which is where I am located.

I'm afraid replacing files does counts as modifying, and thus breaks the EULA you agreed to when you ran the installer. Then again I cannot be sure, because I couldn't locate a copy of the EULA anywhere online.