Currently, there is no format specifically catering for magazine articles (or newspaper articles) in the MLA CSL so it falls to the default option which is just to add the page numbers at the end of the date. This can be changed very easily, but it may impact on some other item types that don't require the colon.

I'm not very familiar with the MLA style so perhaps you could try testing this for a while and see if any other problems arise. This also includes the modifications adamsmith made recently but which haven't yet been uploaded to the style repository (see here: http://forums.zotero.org/discussion/11190/modern-language-association-dates/#Item_0 ).

http://www.box.net/shared/ifvh2d2la4

I guess 95% of all citations are for books, book sections and journal articles, and as people are mostly giving their own unpaid time to write styles for Zotero, they are probably not motivated to write styles that cater for other less common reference types they don't personally use. I guess if your work does require you to frequently reference other types then the onus is on you and people like you to improve the existing style.

Personally, I learned CSL by just plunging in and trying to write a style. The Zotero reference test pane is a great place to tinker with the existing styles and see what changes.
chrome://zotero/content/tools/csledit.xul