Structural20036,
A couple of observations on your question:
1. In some states it is illegal to portray yourself as an engineer if you are not licensed in that state (Nevada for example). Even if the project didn't require a licensed engineer to participate, simply promoting yourself as an engineer, or calling yourself a professional engineer, for a project within that state could technically be illegal.
2. Many states have exclusions for various types/sizes of projects - small residential for instance - where a licensed engineer isn't required. In this sense, perhaps they aren't violating any law.
3. There are ethics codes out there that any licensed engineer is expected to follow. These are not simply suggested rules but many times are actually codified into the engineering practice acts of each state. There were some answers I saw on the site that were definitely questionable as to good engineering practice (see the above example by port125). In this case - providing design services for fee when you don't actually have good knowledge of the structural conditions, loadings, etc. is not practicing to the standard of care.
4. The engineer's first priority is to protect the public health and safety. Item 3 above certainly doesn't do that.