I don't like it either, but it's used generally in airsoft (the term has migrated to these boards from other boards) to denote that you're talking about real guns now.
When discussing airsoft, you can't really refer to the generic term of "guns" simply because airsoft is compared to real firearms all the time, and no one will take the time to use "airsoft guns" when referring to airsoft and "guns" when referring to the real thing. "Real firearms" is too long for most (and apparently too complicated for some), so "real steel" has sprung out and is a slang term. Again, I don't like it, but there are a lot of slang terms I hate that the kiddies use in forums and online chat now.