There is absolutely no evidence that spinach is bad for dogs or will cause them any health problems at all. In fact spinach is quite a nutritious and healthy food to eat because it is so vitamin-rich and able to offer certain antioxidants which both people and animals can benefit from. Although your pet may not necessarily like the taste of spinach, it is still probably not going to be harmful to them like eating chocolate or another type of food.
Those who give their dogs spinach will want to make sure to do so in moderation though so as to avoid upsetting their stomachs. Dogs should be fed a steady diet of dog food with the occasional healthy human food items tossed in for variety.