We didn't find a solution that would allow to add subsidy into a DAG-coin without lowering its security. This can't be treated as a proof, but I hope you'll give me a discount because asymmetry of proofs is very large, i.e. it's hard to prove that unicorns don't exist but is trivial to prove the opposite if you happen to find one.
PS: Miners would compete against each other if subsidy was allowed, moving consensus convergence point beyond the horizon.
EDIT: On the first sight, this looks as an excuse for explaining why Iota went sale route instead of generating the tokens a-la Bitcoin. If you have thought so, spend 5 seconds to look at the design again.
No, I don't see this as an excuse - in fact, I am writing a white paper at the moment which has parallels to Tangle, but it has a mining subsidy and I want to be sure that I've covered all the angles, so the fact you chose to disallow it is important to me.