"Hey guys, there's someone with connection to that illegal group we've always been hunting"
"Screw getting info out of them, let's just make them FORGET EVERYTHING"
...Like, seriously, what kind of stupid logic makes them think erasing memories on the spot is the better idea...
Unless...in trying to interrogate them, they have to reveal some secret they'd rather not...like, say, the brimhats isn't actually doing anything 'bad', just that they're too powerful and so without careful management their spells goes too far, like this case, and the council decides rather than revealing this truth and make the brimhats get all the spotlight, they'd rather hide it and remain in power...