Another way colleges check on "showing interest" is through the emails they send. They can monitor if you open the email or simply delete it without opening it. The more deletes, the less interest, the less likely an admit decision.
If this really is the current game in college admissions, I'm surprised so many kids/parents choose to play.
The root problem in the college admissions process is that yield is taken into account in ranking schools
In the end, it's just a numbers game... schools see overly qualified students as less likely to attend.
What if a kid has limited email access, and learned about the college from a Fiske guide at the local library?
They're trying to craft a community, and the higher the yield the easier this is to do.
A kid who has displayed a low level of interest is less likely to attend, not just because it may be his backup school, but because a student who hasn't done much research on a school is unlikely to be able to assess whether the school is a good fit.