If you spend any time playing with malware, or just downloading software, you've probably seen those kinds of scams.
Take some free software, wrap it in Nullsoft Installer, add a few toolbar and "system optimizer" softwares to the bundle, make the installation dialog as confusing as possible and get commissions for each install.
The reason why it works - people are stupid. They just click "Next", "Next", "Next", "Finish" and think it's gonna be alright. Sorry grandma - you just made somebody a few bucks richer!
These types of installers are usually detected as Adware or PUAs (Potentially Unwanted Programs) by most antivirus companies. The criteria for detection are really simple - if your installation dialog is designed to confuse Average Farmer Joe, you should be detected. You may not hide "Decline" button, you may not try to blend it into background, it must be clearly visible and accessible.
And now look at Elementary OS
Having said that, just look at the new and improved download page for ElementaryOS - freeware, open-source operating system:
Can you see the free download button? Neither can I. Because it's not there!
You have to explicitly click on "$ Custom", enter "0" there, and then click "Download".
Huh? Come again, please?
Apparently, someone at Elementary OS thinks it's a great feature:
We’ve opted to present users with some easy one-button choices. Right now we have ambitious $10, $25, and $50 buttons along with a “Custom” button that lets you type anything—including $0.
We didn’t exclude a $0 button to deceive you; we believe our software really is worth something.
You, sir, are a fucking moron.