Well its happened in the past, Mabnen as I recall involving Blood Inquisition and their subsequent withdrawal from the Bleaklands into Delve. But that was before my time around early 2005. They viral-bombed the planet.
I'm sure it happens that some idiot capsuleer will fire shells at a populated planet for giggles, but most likely CONCORD appears, and royally fucks up the capsuleer and his pod-liscence or what not for using disproportionate force in empire space. But as I recall, capsuleers do bomb planets for faction warfare yea?
More importantly, the question is - to what end?
Its a bit like terrorism really. Buying an AK on the market in europe isn't all that difficult. Hell its easy enough to buy a shotgun for hunting. You could easily go into a primary school and start shooting. But what for? To what end? What does it achieve? What is the intended end state?
We could apply this to capsuleers. First of all, more importantly - if they're complete homicidal psychopaths before capsuleer training, how the hell did they get past the screening processes and psych evaluation? I'm sure a percentage got through, but it should be in theory minimal. I highly doubt people were 'born' capsuleers, but became capsuleers. Yes, they could turn into eventual beings with little morals, but the initial psyche they had when they were still human should still in theory still provide a form of "natural" safeguard against complete psychopathic behavior.
What would a capsuleer bombing a planet to kill someone achieve? Disproportionate use of firepower for a personal objective, using a ship that would easily be spotted and traced. Its even quite possible that CONCORD/Yulai demands a form of ship registration for every ship built and flown. There could also be passive tracking of ships that pass through what systems. So getting away without being spotted by CONCORD (unless in 0.0) would be rather.... difficult.
Also it could be that some weapons are simply NOT on the market and not sold. In the IRL weapons market, officially at the government level, you can buy all sorts of shit. Small arms, armored vehicles, jet fighters, tanks, ballistic missiles. You only have to go to Eurosatory for a day to see what sort of weapons are being sold officially on the world arms market. Its impressive at first, but you soon become a little disturbed as you realize that the small rooms in the corner of the hall is not the restrooms nor rooms to get a little action with the hot stand girls, but rooms to negotiate purchases from big armament corporations like Rhinemetal and MBDA to buy the latest missile technology and launch platform. That said.... some weapons are just not for sale. You will not be able to buy nuclear launch vectors. Chemical weapns are not sold. WMDs are not sold. Cluster bombs are not sold. Mines are not sold. And etc etc. I think likewise in EVE, planet bombardment weapons are kept precisely in the total control of State-powers to avoid the retardery involved with capsuleers getting their hands on them. I'm sure those that do will attempt to use them should they be psychotic enough, but the response would be similar to capsuleers shooting at another capsuleer in high-sec without the proper documents. In null-sec perhaps... but noone (CONCORD) cares, since thats no-man's land.
Third potential reason is that its disproportionate. What is the end-state of bombing a planet? What the hell for? To "pursuade" someone into paying protection money? I'm sure the local planetary government could call up the local navy or CONCORD to blow that capsuleer away and revoke his licence. But still, bombing a planet and killing 5 million people. How does that help your cause in having people actually want to deal with you? Probably very counter-productive. Even in terrorism, going after children is a no-no. PR wise its counter-productive, and does nothing to further your cause. You want people to support your struggles, not turn unilaterally against you for being an uncontrolled psychopath.