There usually isn't that much to fence, i mean some commlinks and some guns, is generally the take away.
Even 30% book value just isn't going to be that much is it?
It all depends. Most of the time the runners won't have the alignment of circumstances and connections to harvest cyberware from slain opposition, or to claim things like security vehicles intact. But in those cases where they do, the SRM fencing rules ensures they don't totally blow the nuyen rewards out of line with what's programmed to be awarded. Even still: getting 30% of half a million nuyen worth of vehicle and/or cyberware is still potentially gonna be worth more than the 10-12 thousand nuyen they get from Mr Johnson/Tanaka-san.
Now granted 25% or 30% in such a case isn't that much more than 20% that's possible under the ceiling of 4 Loyalty, but as I said it's more of the principle of the thing. Why should 25/30% on fencing be possible with Chargen-generated Contacts but never with Contacts gained afterwards? It's minor and perhaps only applicable in corner case looting, but it still seems "wrong". Rather than raising the Loyalty ceiling, it just seems more elegant to disallow Loyalty 5/6 on made-up Contacts from chargen. The extra percentage points on fenced gear is part of why it's more elegant, but also there's no reason to have 5/6 Loyalty besides fencing because Contacts with that much loyalty must necessarily have super low Connection ratings, which in turn means the extra Loyalty is useless (other than to exploit the Fencing rules).
Edit: Perhaps the problem I'm seeing is better fixed by addressing Fencing rather than Loyalty. If Connection becomes important, the abuse/exploit may simply go away. If Connection 1 becomes insufficient to Fence expensive loot, the 6 Loyalty to gain 30% of said loot isn't incentivized.