Let's put it this way, Mufasa had gotten into a nasty fight with some of them only the day before and yet Scar just bring them in. He may be the king, but I'm surprised that he wasn't challenged on this. It seems that Zazu at the very least knew that the hyenas and their poaching were a problem.
More to the point, it seems that Scar says "With a heavy heart I assume the throne." , meaning that that moment was officially when he took the throne. However, the hyenas appear already walking into sight of Pride Rock as he makes his announcement to let the hyenas in.
It seems almost certain that Mufasa had a law banning the hyenas from entering the Pridelands, and, until Scar struck it down upon assuming the throne (I don't think he took the throne until that night during his speech, likely because the afternoon involved investigating the death of Mufasa and the so-called death of Simba), the rule would still be in effect. Hence, it's surprising that nobody tried to forcefully chase them off to enforce Mufasa's rule as they crossed into the Pridelands, which they must have done BEFORE Scar changed the law.