I've done it for a similar reason: it cuts down on support requests (ever so slightly). No matter what you tell them, users will store passwords in text files

I've done it for a similar reason: it cuts down on support requests (ever so slightly). No matter what you tell them, users will store passwords in text files and email them around in spreadsheets, and spaces interfere with cutting and pasting in a way that other characters don't. It's not a strong reason, but no-one's ever complained about not being allowed to use spaces either.