I think this is a relatively recent development... all of the non-local LAN Argentina tickets used to book into Y/B/H, earning full miles and tier points. Pricing for H used to be quite decent, B rather punchy but still not ridiculous, and Y usually quite ridiculous.

For some reason, now they have a fourth level, M, which is more expensive than H ever used to be, and as you point out earns zilch in BAEC. H, B and Y are all still for sale, but even more expensive.

Frankly it's all enough to make me consider trying AR again for domestic flights, and credit to Delta or AF now that they're in SkyTeam. I never thought LA could make both their fares and their earning rates so unattractive as to make me consider that!