This is an idea the US carriers came up with a little while ago, and they're the only ones that do it.

Outside of the US, you pays your money, and you takes your chances. Either the fare holds, or increases, or it drops. If the airline puts additional seats on sale at a lower fare later, why should they give you back the money they managed to convince you to pay in the first place? How is that in their interest?

Sale fares or reduced prices are an incentive to attract customers they would not have gotten otherwise - and they obviously to buy even without the lower price. You're not part of the target group for the new fare.

If a supermarket sells you a bottle of wine for $15, and tomorrow they have it on sale for $12, you can't claim the difference there either.

Go ahead, call LX - my guess is they'll have a good laugh about it and tell you to shove off (in slightly friendlier terms, of course).