Why is Adobe recommending a McAfee security scan during the installation of flash player?
Adobe is installing a highly limited version of McAfee antivirus on your system and then doing a limited scan to possibly find malware (as expected with limited software). Adobe also pushes the Bing toolbar, the Google toolbar and the Google Chrome Browser at you among other offerings they've included. Funny thing is their installer routine is not intelligent enough to check for prior installations before recommending any of this software.
While there are good intentions in recommending this software, (McAfee for people with no antivirus at all might be a good thing, the Chrome Browser might be more secure than Internet Explorer) it's mostly done as an affiliate kickback program to provide a revenue stream for Adobe's "free" Flash plugins and Acrobat Reader.
It's not totally harmless as the McAfee scanner has been incompatible with already installed competitor's antivirus packages, sometimes knocking them out of action or preventing their virus signature updates from working. Also, reinstalling Chrome on a system that already has it installed can mess the Chrome Browser install up to the point I've had to reinstall Chrome.
When you manually trigger Adobe Flash, Adobe Reader and Oracle Java updates, you can tell the upgrade to not include these affiliate packages. They also can be removed through Windows add/remove programs routines if you have problems with them. As they are legitimate software annoyingly installed, they follow the rules for uninstall.
I just updated Adobe's Flash Player and it didn't even offer me the option NOT to install the McAfee junk. Luckily I have a program called "WinPatrol" installed, which alerts me to anything attempting to install itself in my Start-Ups and gives me the option to say NO. Then I can simply uninstall the junk.
As Polynomial says, there is that affiliation - many companies promote other installs when you update or install a package. It is pretty common business.
That said, it is always a good idea to scan any new software that you install. If you already have a good A/V or antimalware solution, stick with that, otherwise I would recommend getting one.