The "spirit" of the rules is that more overtaking & hence better racing was needed. I think everyone understood this. The D3 teams engineers were just able to design their cars without being skewed by discussion had in OWG meetings.
To call them cheaters is perhaps a bit strong.
What loopholes & grey areas would result from the OWG technical meetings & subsequent technical specs, could not have been predicted in the detail needed to gain any loophole/grey area advantages. The D3 therefore, would not have had the motives you suggest as reasons not to participate.
I don't understand why you think they should be penalised even though you agree that the diffusers look legal (as in loophole was exploited rather than design being outside of regs).
You also agree that there are always grey areas to exploit... well here are three teams exploiting those areas & people complain.
Your point about how the specs being too ambiguous is correct. It probably has to do with FIA & FOA plans to wrestle control from the teams... but that's another discussion altogether!