That was very bad service and advice,... :( No, the front brakes on the IS are NOT a bad design. However, there was a recall for the rear brakes seazing and they are covered. If your brakes were not squealing before the change to the low dust pads, then they were not installed following correct procedures. i.e. new shims and fresh lube.
Try this. find a remote road safe from traffic and accelerate to about 60 mph and apply the brakes hard but DO NOT come to a complete stop, keep driving. Repeat this three times, 60 mph, heavy braking, no stopping,...keeep moving. then drive the car normally to allow the brakes to cool down.
Once you've completed these steps see if the brakes still squeal? If they do,...order neww shims from Sewell Lexus,...or take it there if it's not too far away for service.