It is implicit in this term 'clean' that any edibles which fall outside this category are what, dirty? Whether or not we chose to pay any heed to it, or act as though we care, each and every one of us is aware that there are certain foods which are designed to be eaten in moderation and are not particularly good for us. I will save you the trauma of enumerating them for this very reason. However, I firmly believe that labelling a way of eating, whether salubrious or not, can potentially be very dangerous. No foods, nutritious or not, should not be excluded as untouchable or worthy of absolute exclusion from your diet. This ingrains in society the idea that food should be used as a gauge for determining your self-worth - whether you are virtuous and eat 'clean' or whether you are malevolent for eating... dirty? Nobody speaks about eating foods equated to dirt, and yet that is what is constantly being implied when you fail to live on kale and quinoa (I'm not judging their moderate consumption; quinoa is an adored staple in my larder and I have no contention with leafy greens).
A part of these healthy eating rules -that is in effect what they have become - appears to be the notion that it is a good idea to use food as a reward or something which has to be justified. Carbohydrates have returned from exile (phew) but unfortunately still remain a food group which seems to necessitate an intense workout before you can eat them. These ideas of 'perfect' eating and 'work=reward' encapsulate one of the greatest issues surrounding societal relationships with food - imbalance. Eating purely 'good' foods will not make you healthy, and eating only 'bad' foods certainly won't. Making the distinction between a health-giving, beneficial diet, which includes the occasional unhealthy indulgence, and an obsession with a 'perfect' way of eating is the only way to truly healthy diet.