Most modern browsers don't really show any variables passed to the website through the url if it's after the question mark sign and you can also easily shorten your urls. It wouldn't be hard to implement referral tracking with a url shortener that could be built right withing steemit.com. I believe the benefits far outweigh the possible problems

On top of this, the fact that traffic to individual pieces of content from people not logged in (or even ones that are logged in?) has not bearing on the rewards means that people have zero incentive to share their steemit content outside the platform.