The fare rules are on Lan.com. When I get the fare matrix after selecting city pairs and dates, the matrix starts with Base on the left, then Base Plus, then Flex, Full Flex, and so on.
When I roll my mouse over 'Base' it states:
Does not allow advance seat reservation. You can choose your seat from those available 48 hours before the flight by checking in online at Lan.com
When I roll my mouse over 'Base Plus' the pop up window states:
Code:
Allows for advance seat reservation
I could not locate any exceptions to those rules (such as no seat assignments to/from the USA) on their website.
So, unless everyone purchased 'Base' and nobody purchased any higher fares, then you're correct. However, for anyone that purchases Base Plus, their website states that advance seat selection is available.
Your experience is intriguing, however. If you are seeing blank seat maps with wide-open availability, then I'm not sure what to make of that.