And even if, as you say, it is "for work", why does Google force that decision upon me, the owner/operator/admin/sole employee of my one man organization? Why can't it just be a check box that says "I really just want a custom domain. Please let me use the cool things you make"?

It's infuriating. I have no problem paying $5/month just for a custom domain on a standard Gmail account, but it's not possible. This is the only other option, and if I had known how much frustration it would cause I'd never have used my Apps account as my primary for the past years as I now have numerous purchases, domains, email archives, photos, and so on attached to it.

Probably because it's not just a matter of marking it enabled. For example, I wouldn't be surprised if they used two separated branches of the codebase, only merging features from regular services into Apps after they are more stable.