If I’m analyzing this correctly, every spider drone used by an infiltrator counts as a soldier for purposes of dividing XP. So it looks like you get less XP for using your class features if you’re an infiltrator. You don’t lose XP because your sniper used a bullet to kill something, so why should you be punished for using a spider drone?
If this is an oversight in the coding, then I’ll happily look forward to it being fixed in an eventual patch. If it’s a conscious design decision, then I urge you to reconsider.