The downside to the "have a transaction on the awarding of a bounty" idea, along with having transactions scale with value for non-murder transactions, is that this breaks the principle that all players are able to influence the BGS. Some guy taking on low-intensity conflict zones in a viper, or bounties in a res, is going to grab one or two kills then bug out and this is fine.
Missions, likewise, can be run in any ship capable of taking them on.
Mass-murder of authority ships, on the other hand, is an activity that very much benefits from having one of the big four. Especially a cutter, which can low-wake away from anything the system authorities will ever send after you, but can keep killing them in swarms right up until the point its impressive shields drop.
Instead of bringing bounty hunting in line with murder (which may tip the scales too far the other way in terms of influence - why run missions when you can park a battlebarge in the high-res with 1t of gold in the hold?) rate-limiting how quickly criminal on-the-spot transactions can be applied seems to make it more consistent with everything else.