If you wouldn't have been motivated by the travel experience from that one long-haul flight in a very long time to have gone out in the last 16 months and spend serious cash on more air travel;
then, your activity doesn't represent a marginal loss of revenue to the airlines. Whereas the suppliers who are awarding you the million+ miles required for such redemptions are paying the airlines' frequent flier programs for those miles, thus representing a marginal gain of revenue.

The magic is yield management -- airlines letting you redeem miles for seats that they don't think they're going to be able to get more revenue for, either from people willing to purchase such seats outright or from upgraders.

(And that's a good reason why the airlines who have unrestricted availability awards for roughly double the number of miles are scratching their heads about whether to keep such awards. Yield management can't prevent people whose spending patterns allow them to earn millions of miles to spend those miles for unrestricted awards, people who might otherwise be willing to actually pay cash for business class or first class seats.)