No Ring, No Bring!

Being a wedding DJ, I hear all kinds of things about planning and guests. The latest is "No Ring, No Bring". Basically, It's a rule when it comes to the wedding invitations. Basically, unless you are engaged or married, you are not allowed to bring a plus one! WHAT? Why invite someone who has a boyfriend/girlfriend and not allow them to bring the significant other along? Is it because the catering is so expensive, or is it something else? Unless It's your maid of honor, bridesmaid, best man, whatever, how do you tell someone that just because they are single, they have to go alone? Is this the new way of weeding your single friends out to start your new life with only married couples?

Messed up or not?