Firstly, it's shaped like bra and panties, only that women can use it on the pool and beach. Second, it can be worn in public.
The issue here is the fact that bikinis cover the same (little) amount of skin as lingerie. Since its invention during the 50s or 60s, it got smaller and smaller. It may be ***y, but aren't the girls wearing this showing too much? Also, if they happen to wear a pure white or black one, it does look pretty much like lingerie from afar, doesn't it?