Seen the comment there about a woman now suffering with plantar fasciitis so did a search to see how often its asked about - The answer is LOADS. And this particular post had 581 ppl replying!!
Because he badgers people to get 15,000 steps in when they aren't used to doing 15,000 daily. Stepping on the spot on hard surfaces especially if a person is overweight can contribute due to the impact. Stepping on the spot is not like walking you have a different range of motion, gait, impact, distribution and reverberation of the impact when walking than stepping on the spot. Stepping on the spot is fine as part of a warm up but to do it for long periods in place of actually walking can for some contribute to this injury. Walking around the house, garden or up and down the street AND gradually building up to the amount of steps you want to do, can do, or can fit in to your day is better than going all out on the spot. You wouldn't go from 10kg loaded squat to 100kg loaded, you wouldn't jump on a treadmill and run 10 mile daily after only ever walking 5 mile 3 times a week. No you build up to it and steps should be the same because despite what other people think not everyone can walk for long periods or distances so need to build up to it.
If you wouldn't tell your children, family, friends to step on the spot or if you would not recommend or question if they were mixing weird food combinations why do it yourself. It should be safe, sustainable, enjoyable and 'normal' for each individual client.