For someone to steal from you, you must have property rights over that object that entitles you the right to exclusive use. The problem with the “inflation is theft” argument is that it requires a claim of property rights to a pecuniary value of an object. However, greenbacks only represent a claim on U.S. goods, services, and assets. They do not represent a claim for you to be able to receive two candy bars or an hour of parking for every dollar you posses.
Free marketer’s do not usually make this error in other domains. If someone were to claim a right to be paid as much or more for their house as what they paid for it, they would be correctly reminded that their purchase only granted them rights to the house and property, not to any pecuniary value of the home. In my view, the "inflation is theft" crowd make this error.