why wouldn't you be grandfathered in the way people were on the baggage policy who bought their tickets before may 08th(or whatever the date was for avoiding the fee) but who were traveling in july?

two months ago i purchased about 12,000 miles of lan flights in discount economy for travel after june 1st and there was no mention of an impending change. i thought i would be getting 100% of the miles and frankly, had i known, i would have chosen flights with american or with taca/avianca--and saved some money! these flights were going to get me status ( not to mention seats on gorgeous, comfortable 767's with edible meals, fantastic personal video screens, and most importantly, humane seat pitch!)

i know they say that the mileage rules can change at anytime but don't they legally have to honor the rules in place at the time of purchase. now that i know the rules i'll plan accordingly--i don't think the new policy is unfair (it's annoying but it is what it is)--but i need those miles! should i write a letter? do i have any legitimate grounds for complaint?