User blog comment:Hawki/Nova Covert Ops, part 3 - Reflections/@comment-4817-20161130120308/@comment-250398-20161201045804

Purification is a weird thing in that it's only in the story when it doesn't wreck everything. I mean the protoss fleet had several opportunities to purify Char before where there seemed to little contesting them in orbit, the time when they retrieved the Khalis in Brood War comes to mind. So far I think they've done the best option and avoided talking about when/where they're able to purify planets. I mean hell Mar Sara and Antiga Prime's space seemed very contested and they were able to do it then.

Having said that I also doubt the Tal'darim didn't have the technology for it, they seemed fine with doing a similar glassing to Glacius (though at a much smaller scale albeit) and when the Daelaam returns to Aiur Alarak's first suggestion is to glass the planet. Considering the Tal'darim's idea of changing the normally peaceful protoss technology into better war machines I feel like they'd have something similar to purification technology. Then again that would explain why Amon valued the Golden Armada so much more than the Tal'darim.

But yeah when/where the protoss can purify planets is one of those things that opens a whole can of worms, and they've thankfully sidestepped because there really isn't a good reason a lot of key battles couldn't have been fixed by purifying the planet. Most you can argue the morality of the protoss or not wanting to harm the terrans there, but also none of that really flies with super infested worlds like Char and the desperation they went through after losing Aiur.