They don't force anyone to sign up. If you do nothing other than fixing your website, eventually Google will check it again and remove from blacklist.
Seriously, what's your complaint? If you don't want to get blacklisted, don't let your site be hacked. If your site is hacked, and you're complaining that Google blacklisted it and notified you about it, you're dumb.
And guess what -- they provide this service (and also pay the real person to review your re-listing request) for FREE.
>And after that, it forces the owners of the site to register with Google and use Google services just to even figure out why, and to get their sites unflagged.
Google forces you to prove that you own the domain before they give you any information that they don't release publicly. How else do you suggest they go about not releasing everything publicly? Also, all you have to do as a site owner is click on the safe browsing diagnostic link and go from there.
And after that, it forces the owners of the site to register with Google and use Google services just to even figure out why, and to get their sites unflagged. And that is after the owner even figured out how and where to contact Google.