One big contributor to plantar fasciitis is the little structureless ballet pumps and overly squishy ‘trainers’ we are all prone to wearing. Plantar Fasciitis is a symptom of over use/over stretching and when our feet get no support everything overworks and overstretches. That is not to say we need rock solid but some structure is really important. One of the reasons that heel cups relieve symptoms is that they shorten the structures in the feet so relieve the over use a bit.
Bear in mind that we are all different too and some peoples feet are better with less structure – a good podiatrist will get you right