Bitcoin mining is legal and is accomplished by running SHA256 double round hash verification processes in order to validate Bitcoin transactions and provide the requisite security for the public ledger of the Bitcoin network.
If the trade-offs here feel similar to the trade-offs between relational databases and no-sql databases, it's because they are. At the end of the day, all of these technologies are built on top of the same well studied data structures and the innovation is happening by applying them to new industries.
The only definition of rationality that I found that is practically, empirically, and mathematically rigorous is that of survival –and indeed, unlike the modern theories by psychosophasters, it maps to the classics. Anything that hinders one’s survival at an individual, collective, tribal, or general level is deemed irrational.