It's surprising isn't it. I can't say I miss an MS Paint level program what with having Photoshop there (although I'm trying to ditch it) but I'm surprised Apple haven't done a basic Mac Paint program for OSX. I'm sure there's plenty of free equivalents though on macupdate.com
I've often though Apple could do themselves a favour by picking up Intaglio (an OSX equivalent of MacDraw) and including it in iWork and I'd think there would be a MacPaint equivalent too but I've just not looked.
the original Mac had a paint program. I would think that a slightly revamped Preview.app with simple options for brush strokes, shapes, etc would be a welcome addition. As it stands right now, I can only recall using Windows on Parallels for MS Paint and some .EXE Cisco simulators.