It's a mental "snap-to-grid" problem. The average American hears the word "free", and they immediately think of $0, absent a specific pre-existing linguistic context (free country, free speech, free range). Because software is something that is frequently paid for as a product on a shelf, a non-enthusiast is always going to hear the $0 meaning in their head, even if they've been educated as to its other meaning.

At a high layer of abstraction, neural connections work probabilistically, firing in the path most likely to be a useful pattern; hence, most people will tend to assume "free" means "as in beer". However, a new term like "liberated software" will not trip this pre-existing wiring, forcing a new pathway that doesn't have to compete with the conditioning created by a lifetime of marketing messages ("Free Checking! Free Estimate! Free For The First Year!"... etc)