Yes, this tax proposal seems to presume that the mining being done is a value-adding operation. However, an adversary desiring to undermine the electrical grid

Yes, this tax proposal seems to presume that the mining being done is a value-adding operation. However, an adversary desiring to undermine the electrical grid could just set up servers sending back cryptographic proof that they executed some useless inner loop. Who even knows with all the various crypto-coins now.