Why does the application of laws depend on status?

We are strictly speaking about foreign laws being applied to a given, perhaps formerly, sovereign nation.

Things that happen to someone with high status have different results in regard the local populations as well as foreign.

If you have a policy of killing respected conqured leaders, not only will this grow resentment from conqured nations, but other nations are less likely to surrender. Among other political issues.

If you always show mercy this encourages people to surrender, but may also embolden rebelious types to think they can surrender if they fail.

Take a random group of low status people and cruicify them and a bunch of people may be upset for a bit, but the greater political landscape doesn't care.

Also, time, money, and effort involved with going after individuals of a conqured nation may not be worth it.

High status individuals are few, what happens to them have wide ranging consequences, and they tend to be more efficient targets.

Really though, that applies everywhere, even outside our specific situation.