Rashad got hurt and will be out for a significant time, its not a 2 week recovery period. Dana White needed a replacement and he basically gave JBJ no way of turning it down even if he wanted to, he told Joe Rogan to go in there right after a win and offer him a shot.
What is JBJ supposed to do on live TV? say he doesn't want it because Rashad is next in line after his recovery? that is detrimental to his career and will probably lose some fans of his.
Rashad had his chance at the belt, its not like he will be forced to move down to MW without ever getting a shot at the LHW title. If Rashad never had the opportunity to fight for the title, I could see how JBJ could have said he wants Rashad to make a run for the belt first since he is older, but even then its sketchy in this sport.
Dana White hates when fighters don't fight their team, it would even make him more angry if one of his next biggest stars declinded a title when no team member is currently holding it.