Moron question of the day but.....how do you implement terraforming?
Terraforming is Stripped To The Core, it just goes in reverse. Instead of nuking your planet for resources, you trade a functioning planet for a better one later(Toxics upgrade to Forests with 480 seconds added to the scuttle time) and the lost income is weighed against future gains.
Off topic question; why do most of the planets still have a "Low Infrastructure" penalty at the end of the upgrade line?
Three reasons, pacing, the impact of bombing damage, and aesthetics.
Bombing down an unimproved Asteroid is largely a waste of time, you lose a minor little bit of income and an inexpensive upgrade, rapidly recolonize it, and the damage is swiftly made irrelevant. If you don't succeed in actually killing the planet you just bombed, you've done next to nothing.
Pacing, the early game worth of Asteroids is very, very high in vanilla Sins, they're far more valuable than much better planets that have to be researched. They have a very good Return On Investment, to balance this out a bit and make them less effective, they needed drawbacks.
So, they come with a negative and reduced initial incomes. Some have further negatives from planet bonuses of course, but the planets themselves can balance out and go positive easily enough. They are not a substantial cash supplier though. Later in the game, you get substantial population upgrades, the asteroids become more valuable, so as to not be irrelevant monetarily. Your initial expansion is primarily funded by your home world until you do civilian research. This reduces the viability of an all military strategy where you just colonize asteroids.
There's also the practicality of an asteroid colony not having major costs to maintain. A minor factor, but one supported by game designs. The negative income modifiers represent the cost of maintaining an undeveloped outpost. A thriving economy will supply it's own goods and services, a simple military installation will require supplies for everything. A domed city on a lifeless asteroid requires significant exterior supplies to maintain it's existence, civil population or not. All of the planets that maintain negative modifiers are harsh environments, lacking in natural resources in a way that would require a great deal of supplies, or more complicated means of compensation, such as the need to import most of your food supply in a harsh desert climate, with extreme water recycling measures needed to keep the plumbing operational without additional off world input.
Some planets are just bad, you have to really want that Toxic. It's a nasty place to colonize. It's a good point of defense, it has nice resources you need, you can terraform it, you can afford to wait for it to pay itself off with trade ports later, etcetera.