Well there is...
In the first instance, a STHer is actively purchasing tickets for guests and agreeing to T&Cs whilst doing so. The club can limit how many can do this, and for which games, and has a record of exactly when it happens.
If you pass on a ticket to a random, it's a casual...
The guest tickets thing (and the club not knowing who is sitting in the seat) is definitely an anomaly, but still managed by the club.
That's different to allowing, say, 2000+ Albion fans being free to distribute their Man United or Chelsea tickets as they wish.
Honestly, can you imagine...
It's legal to transfer football tickets with the issuing club's permission. Hence why there's an official ticket share system.
There are, admitedly, some anomalies in Brighton's system - eg the ability for STHers to buy additional tickets for randoms for some matches, or to upgrade ticket...