Under the current system, the Lawyers do (aka politicians). Thats why they love an opaque tax code - exemptions can be sold to donors and given out to curry votes. Politicians love to play social engineer.
I say fuck all the exemptions. No exemptions. Everyone pays. Make it simple. We allow some exemptions, it's the camels nose under the tent. In ten years, there's an exemption for every industry lobby group and every one else gets fucked. Same as it is now. System can't be pay-for-exemption. It's gotta be aportioned like the Founders said - everyone pays the same rate. Fuck them all.