"HBO will have its due."
As is traditional for American TV shows, the original cast contract for Thrones was for six seasons, potentially with an option for a seventh. Hollywood contract law prevents actors from being signed up for longer periods without renegotiations. It was anticipated that such contract negotiations would take place a year or so further down the line, but HBO has clearly taken the line that it wants any possible conflicts resolved earlier, allowing them to greenlight (at least unofficially) a seventh season far ahead of writing and shooting. The move is likely expensive for HBO, as it means the raise takes effect from the fifth season rather than just the seventh, but also may reduce the salary demands that would have come with waiting longer. Several Thrones castmembers are on big movie projects and may have held up for much more substantial raises for the final season if those projects had taken off, such as Emilia Clarke's role as Sarah Connor on the new Terminator movie trilogy (although if it's half as horrible as the plot sounds, maybe HBO jumped the gun a bit here).
No mention is made of an eighth season, which may just mean that it was unofficially discussed but more likely means that Thrones will end after its seventh year. This decision was already made some time ago, although the producers later allowed that an eighth season might still be possible. However, this salary news combined with the news that showrunners David Benioff and D.B. Weiss are already lining up post-GoT projects and the apparently ruthless disposing of major plot elements in the fifth season from the books (where it appears several entire major storylines and subplots from the novels have been jettisoned in their entirety) to allow a faster wrap-up, all heavily indicate that Game of Thrones will end with the conclusion of its seventh season, in just two and a half years.