I know that there is a checkbox to select to include the security scan but I honestly believe that you have deliberately layed out the page in a way that would cause most to miss this checkbox. I believe you are familiar with Gestalt Principles and know that people naturally deal with separate columns of text as seprate items. I believe you are taking advantage of this to maximize the number of people who unknowingly install Security Scan along with their Adobe reader.
I disagree with the idea of including the extra software by default and wish you would stop. If you insist on continuing with that, please change the download page layout to make it more clear that you are getting software you don't want along with the software you are looking for.
We're simply fellow users like yourself so can't do anything about this.
BUT, it is deceptive and if it helps, many people agree with you on this (including myself). At best, they should have the check box set as an opt-in rather than opt-out.
There are a few threads going in the forum that is geared more towards the adobe.com site at http://forums.adobe.com/community/general/adobedotcom_feedback. You can join in on any one of them. It is also more prone to being seen by someone that can make a change.