An interesting point.  I suppose one could argue that upon contact with solid matter, the laser's energy turns violently into heat, causing, in effect, an explosion.  Or it could be an energy weapon similar to what was used by the aliens in the movie Independence Day.

As for your last question.  I don't build siege frigates (TEC is all I play).  A few capitol ships do just fine for a planet.  Siege frigates take too much fleet supply for what they do.  I'd rather build a carrier.

The only faction that really has much use for siege frigates is Advent.  Both TEC and Vasari have excellent bombardment specialist capital ships, so you're almost always better just getting one of those.  Advent lacks a good bombardment capital ship, so sometimes when you need to break through a 6000 hp planet you may want to bring them in to speed things up.

Siege frigates just cost too much and are useless in battle.  What's the point of being able to quickly raze a planet if you can't defeat the enemy fleet guarding it, after all.

what if the Vasari siege beams aren't actually targetting buildings or anything like that.

what if they're drilling gigantic holes in the crust of the planet which would result in rampant Volcanic eruptions, earthquakes, and tidal waves. how's that for matching a Nuke in destructive power?

I don't really know what a 'kinetic bolt' is supposed to be... but some things that may justify the status quo are

- it's not really slaughtering everybody, but destabilizing the government so they take over (unless we assume that the residents are *really* fast at...  procreation, it's not likely that a Terran planet can go from zero to maximum population during even a fairly long war...)

- that might mean destroying or threatening to destroy critical infrastructure.  A high-technology society with a specialist economy for efficiency is going to be fairly vulnerable (threating power generation facilities, for instance, or crucial parts of a transport grid) compared to dispersed subsistence agriculture or the like, and isn't going to like enduring a bombardment that wouldn't actually directly kill them all but might trigger mass starvation, general breakdown and the like.

- projectiles like nuclear weapons have lower velocity than laser beams, and might be easier to interfere with by shooting down (and nuclear weapons are somewhat finicky things)

You don't have to kill the entire population. Why would you do that, when the population is better left alive and working for you? Instead, the primary goal of the siege is to wipe out the planetary government (like targetting the capital, military bases, police stations, civic structures), destroying the government's ability to maintain control, while leaving the civilians intact. The Vasari lasers allow for precision strikes on strategic structures, create loads of EM radiation to shut down communications, burn out computer systems etc., and can burrow deep into planetary crust to take out underground facilities. They get population bonus techs because they didn't have to wipe out half the "real" population in the process. All the more slaves for the Vasari to use.

The Advent use a combination of mind altering aerial drugs and hypnotic suggestion to destabilize the populace and government from the inside out. That's why their missiles are blue!

TEC just blow it all to hell. Nukes nukes nukes. Surrender and/or die, it's your choice really. Very effective at negotiating a change of government.

the advent were beta testing final judgement. they've buffed it now.

Sometimes I build siege frigates when playing Advent.  It is cheaper than Revelation and does more damage pound for pound.  Revelation fights better and has Reverie (good counter to Marza).  The draw back is that siege has less hp than even light frigates.