A reason that is not explained, so I'm going to assume is something petty. There are a couple of instances of sites doing this (for HN and other referral sources) because people said things they didn't like about them (or just in general) on the platform.

Rather than a pissy referrer filter, why not redirect to a page full of ads.

Which is pissier: the filter, or the complaints about it?

100% my complaint