MS is certainly within their rights, but the fact that Linux users are using MS Office, instead of Open Office, would be something I would think MS would play up, rather than antagonize those who have purchased valid copies of MS Office, regardless of what platform they run it in.
It doesn't really sound like it will be that big a deal in the end, though. How many people can this really affect? Not more than a handful, I would think.
My personal opinion is that MS Office is a just a little more polished, and a little less buggy than Open Office. Obviously some share of Linux users feel the same way, or this wouldn't even be an issue. (yes, I do run Linux for some things, but I spend the majority of my computing time in Windows)