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.
Good reason to be careful. Maybe there's a bit of an upside to: if you vouch for someone who does good work, then you get a little boost too. It's how personal relationships work anyway.
----------
I'm pretty skeptical of all things cryptocurrency, but I've wondered if something like this would be an actually good use case of blockchain tech…