Here's how employers could easily skirt the law.
1. Advertise for a position with a very long laundry list of specific technical requirements. (Ever seen that?)
2. Claim there are no qualified workers that exactly match those requirements. (Often ignoring the fact that some of the skills listed could be picked up in weeks).
3. Contact foreign workers who also may not meet the the requirements, then adjust the requirements since no one meets them.
Or, do step 3 first, find specific foreign candidates and get a list of their skills/experience. Then advertise the specific combination of skills/experience for each candidate(s) you are interested in. If you get specific enough. "3 yrs VB, 2 years C++, on Unix, using someUnique software, etc" the odds are very few if any Americans would meet the requirements.
When it comes to wages, games can played with how you classify the programmer. Is he a COBOL programmer that knows
Java, or a Java programmer that knows COBOL? Big difference in "prevailing wages" for the two. Use your imagination and you can come up with better examples...