This is similar to real life: if you vouch for someone (in business for example), and they scam them, your own reputation suffers. So vouching carries risk. Similarly, if you going around someone is unreliable, but people find out they actually aren't, your reputation also suffers. If vouching or denouncing become free, it will become too easy to weaponize.
Then again, if this is the case, why would you risk your own reputation to vouch for anyone anyway.
This is a graph search. If the person you’re evaluating vouches for people those you vouch for denounce, then even if they aren’t denounced per se, you have gained information about how trustworthy you would find that person. (Same in reverse. If they vouch for people who your vouchers vouch for, that indirectly suggests trust even if they aren’t directly vouched for.)