"There's the rub!"
The top part of the "2nd Cousin" category on Ancestry DNA - down to somewhere around 350cM - says the "Predicted Relationship" is "1st-2nd Cousin". The highest 2C I've run into so far is 364cM, so matches in that range are generally 1C1Rs (1st Cousin, once removed).
In fact, the AVERAGE 1C1R is a bit over 400cM, so over HALF of the 1C1Rs will likely show up there.
The thing is, as far as cM numbers go, a half-1C and a 1C1R are the same thing - you share the same amount of DNA with both relations, on average. So over half of all half-1Cs would show up there, in the range above about 350cM that says "1st-2nd Cousin".
**** Which means that most biological half-1Cs could be used to do a DNA confirmation of BOTH your grandparents on that side, if the paper trail says that they're full-1Cs, even thought they're only descended from ONE **** In other words, DNA confirming an ancestor without any DNA from that ancestor. Not good.
So obviously, it's a system with a flaw (probably more than one flaw). In this case, what you could do to fix it would be to have a certain threshhold (for cM, for segments, or for both). That would make things more complicated I suppose (but in some ways it might be come LESS complicated, and just generally make more sense).
This sort of thing is talked about here periodically, so people KNOW it's not perfect. Small changes occur VERY infrequently. It's also been suggested that it be automated to the extent possible (which makes a LOT of sense).
The bottom line is that what we can do about it is (1) talk about it in places like this, hoping it will get around to somebody in a position to work on it, (2) demand our money back and (3) most of all, do your own "DNA Confirmation" with some common sense. I've managed to find applicable DNA matches to confirm 7 out of 8 pairs of my gt-gt grandparents (for the last, there are no 3Cs to match!), but more importantly, the shared matches lists show all the people who are supposed to be on a given branch tend to match each other, and I have all kinds of matches that are 4Cs, 5Cs, and beyond corresponding to most of those gt-gt grandparents. My confidence in my biological lineage really isn't even based on the WikiTree rules! Don't solely rely on them (although they DO work reasonably well most of the time)!