AFAIK it's a practice that goes on throughout the industry. Very rarely will you find the tickets for the same class for the same flight are constant. Indeed, some seem to vary even by the hour on some airlines.
I don't see how this can be illegal, you paid a sum of money for the ticket. If someone else purchases the same ticket at some later stage for a different amount, what is it to you?
I guess you don't want to know how cheap I got my Silverjet tickets then!
Hey, just thought about it, I bought a new camera last month. Nikon are now offering £60 cashback this month on the same camera. Is that illegal?
Or my TV, only a month or so old, is now £100 less than I paid. Is that illegal?