The ZK tax is 35% of the base fare and some but not all of the taxes (normally YQ).
Miles are awarded completely irrespective of the way the ticket was purchased. As long as the airline is advised - whether by the travel agent, or directly by the passenger - of the passenger's frequent flyer number, the miles will be credited (if the itinerary qualifies for miles).
Re the argument re legality - the reason everyone is upset is because there is a discrepancy between what everyone here understands is the real reason for the tax (= to recoup the difference between the price the BCRA is obliged to pay the airlines for their pesos and the price of the dollar on the free market) and the reason stated by the government (to combat evasion of other taxes etc etc, by charging - completely by coincidence - the exact difference between those rates). According to the obvious reason, charging the tax on something paid in dollars is a non-starter; according to the stated reason, there is room to assume that people here are residents. Either way, charging the tax on dollar purchases by nonresidents is rather hard to justify.
The easiest way out of the situation is to either issue the ticket in dollars with a non-Argentine travel agent, who would not charge ZK, or to purchase the ticket in pesos, which a travel agent can normally arrange even in case of non-residents. I work with a travel agency that can do this.