Firstly, YC Reject is, by its very nature, aiming lower than YC. Secondly, no offense to people behind YC Reject, but I would expect YC to be significantly better at spotting the best applicants.
Now, I don't have a startup looking for funding, and if I did I wouldn't expect to get into either. But I'd expect to have a much better chance at getting into YCR than YC. As such, were I looking for a small amount of funding, I might look at YC and think "why bother, not a chance" and then see YCR and think "let's pretend I applied to YC, then I actually have a chance at this one!"
And sure, the cost of applying to each is just filling out an application, and so in future I (the hypothetical me that is in a startup) might apply to YC, knowing that the worst case scenario is I become eligible for YCR. But right now, even the time spent filling out an application is a waste of time if you don't think you can get accepted - but perhaps less likely to be a waste of time applying for YCR.
As to why people would want to lie to apply to YCR - a part of the reason for YCR, and for possible funding, is the PG quotes saying "we are always going to miss some of the best..." etc, and the idea that teams applying to YC, given they thought they might be good enough, they are more likely to be good than teams who thought they definitely wouldn't be good enough for YC. Without the link to YC, there would never be a funding offering based on a blog post saying "let's get a group of startups together to form a network of contacts etc.", after all.