agree that this is a sleazy business practice, but (and not mentioned here, also, not a laywyer but), in music for example, there are typically 'artistic' rights and 'mechanical' rights - e.g. BMG can have the right to duplicate the recording of your song but not to record someone else performing it, or vice-versa where they cannot reproduce your recording but can record others performing it.
There are also nebulous cases involving sampling - some decided in favor of the one sampling, the others decided in favor of those who were sampled, depending on the particulars of the case / district / judge, etc.
I would suspect that getty is taking the public domain image, tweaking the file slightly (perhaps even in non-image related metadata), and thus creating a 'new work' to which they can make some sort of claim about.
so, while i think you are correct in that the 'visual image' (e.g. the visual object representation) is entirely in the public domain, the 'image' (e.g. the actual file) that they actually distribute is not, because they have modified it somehow and 'added value', etc etc etc. then, enforcement just becomes a fishing expedition. if you did your homework and used the public domain one and can refer to it, they don't press. if you didn't, and don't catch it, they have a reasonable claim/plausible deniability of mis-attribution, and you pay. easy (but unethical) money to them.
shady, dubious, etc, but probably not out-and-out fraud in the 'illegal' sense. still not good otherwise because it does facillitate them gaining market share (by having a bigger product base / collection) which in turn creates a cyclical effect/ barriers to entry/ etc (just like BMI/ASCAP and music, for example)