I dont think that the rebuy costs play any role when it comes to this unsportsman like behaviour called "combat outlogging". Most players doing that have big ships and millions more to cover the costs. It's about to not give the winner of the fight the satisfaction of having beaten their opponents, thus destruction of their ships. Good sportsmanship is rare in today's societys.
Well, I think there are two reasons why people "combat log". One is, as you say, pure, simple, unsportsmanlike behaviour. Particularly those who go out of their way to pick a fight and "log" when they realise they aren't going to win. These people could be tackled by more aggressive NPC law enforcement, which grows more aggressive as their murder count increases. If they also lost their ship when they log, if tackled by an NPC cop, all the better.
The other is when people who have no real interest in PvP get attacked by one of the above, usually in a much more powerful ship and don't want to play that game. Those people would be much less likely to "log" if the penalty for losing their ship, particularly in a no-win situation, wasn't so steep.
This is why I said, earlier, that the need for private groups, or even solo mode, was linked to the imbalance between the penalties dished out to the murdered and murderer. Currently the penalty for being murdered is worse than that for being a mass murderer. That is wrong, IMO.