Humans could probably one day become similar to the race described here really, it depends on when the honor code came up. If we transition to post scarcity when we reach for the stars, then we enter a phase where money isn't the motivator. If money isn't, what is? Enter honor. I think that's the key thing you can focus on here, this honor code could develop late into the races history as a response to post scarcity society's evolution (We currently say 'he who dies with the most toys', they say 'he who dies with the most honor'...though in any event the line ends with 'still dies')
If a species has nanobots that can reconfigure a planet at whim, then there really isn't that much of a need to attack another race (I think you can say they are post scarcity if they can rearrange matter as needed)...there's an absolute abundance of other planets, moons, asteroids, and any other manner of matter floating around space for them to use. They are so well advanced that their opponents don't offer much of a threat against them...so whats the reason they are even attacking other species? The only parallel I can take from current earth is the same drive that makes a human go around earth shooting animals on safari (In lost world, saying "I hunted the biggest Dino" is enough to risk ones self)...it's not because there is a need to, but because it's sport. It's gathering experiences, fame, pleasure, honor, and whatever else one gains in killing a creature that can't really defend itself from you brings.
And there's my answer to your question...they've reached the point in their development where they are not attacking for land or material gain, nor are they fighting for their survival...they are fighting for sport. Sport has rules...if a golfer picked up his golf ball and dropped it in the hole, he has infact accomplished the aim of the game (ball was sunk, no?), but there is no glory in it because he didn't do it according to the rules. I'm sure a MMA fighter could win if he took a gun into the octagon as well. Killing a lesser species in a manner other than hand to hand combat is the same type of cheating as picking up and walking around your golf ball. If you're going to play the sport you've gotta play by the rules.
I find it's humorous that the potential of getting caught cheating on an invasion is a possibility...your alien race invades a planet and a few of the soldiers bring in advanced rifles and slaughter everything. This is investigated by the alien species 'invasion commissioner' who determines that heavy plasma weapons were used which results in the disgrace of the invaders under the scandal name 'plasma-gate'.
Incidentally...don't under-estimate the humans race ability to defend ourselves. Guerrilla warfare is a very valid technique vs a technologically superior species and even if your alien species can kill off every tank and plane on earth prior to the invasion, they are going to find a very warlike human race hiding in their home terrain and resisting in every manner possible. Your aliens are going to be in trouble if they can't resist heavy projectile fire, explosives such as RPG's, anti-tank weapons, and even nuclear devices. Like it or not, our current human race is warlike to it's core...there's a good chance the invading aliens with swords might be facing prop planes loaded with explosives and manned by a suicidal pilot wanting to take as much down with them as they can.
Edit as suggested from dsollen - One would have to assume with how far this race has come that the simple destruction of their body/physical form isn't necessarily death to them (otherwise the sport seems pretty risky)...they've got nanomachines and can rearrange matter as is, restoring consciousness from a single surviving cell (5th element style?) or copies/clones or avatar style combat is all a method around this.
You almost have to assume that if they're coming in with swords when they could simply convert the planet with nanobots (us included) to whatever they felt like, that they must be doing it for some form of entertainment/pleasure (be it from actual combat or the 'glory'/honor afterwards) over practicality. Isn't that the background in AvP anyway? The bigger of a resistance we mount is simply more honor in combat to be found for them