It is possible to correct entries in MyFitnessPal, but not easily, as you say. I sometimes come across entries that are 100 times off, so someone (a user, or pulled from a database) has entered e.g. 600 kcal for 1 g when it should clearly be per 100 g.

That means I can either enter having eaten 0.4 of 1 gram, or try to correct the information. But when you try to correct it, you can't just say "multiply everything by 100" or add a new 100 g option. It's only possible to divide all entered macro and micro nutrients by 100 to get to the value per 1 g, which is tedious to do while cooking dinner for the family.