Rice cakes are considered bad for you because rice cakes are not really nutritious so you're really not getting any health benefits from eating the rice cakes.
The rice that is used to make rice cakes is a bit nutritious but it loses it's nutritious value when the rice cakes are made from that rice.
Rice cakes are also low in saturated fats and sugar depending on the variety purchased so that makes then a bit healthy but they are also not the healthiest food you can eat.
The rice cakes are also tasteless and some people including myself find them not pleasant to eat and they taste like cardboard mostly.
Rice cakes are made from refined rice that has very little nutritional value although rice cakes are usually much healthier for you than chips and other non healthy snacks.