The loan rule applies to any club that needs players, not just based on injuries. They just need to be back to their original club before the finals. The team taking the loan players pays their salary, and the players themselves can be recalled earlier.
No rules are broken. Bulldogs are just smart. Its in their best interest to finish the season off on a high, despite not figuring in the finals. There will undoubtedly be other nrl clubs looking at loan players too. Its a good thing IMO. It gives players who are outside of their club's respective 17 match fitness. Both clubs benefit. Players benefit as well by showcasing their ability in the main arena infront of other possible suitors if they are offcontract etc.