He gave them so much free labor
In mid 2000s these companies were already operating in the billions and their engineers were already well compensated, and it was known.
Hell, "Cracking the Coding Interview" came out in 2008. Getting a job at those companies at the time was already something coveted because of how well they paid.
Perhaps in the USA, but in many other countries this does for sure not hold.
I'm sure I could get away with it and nobody at Amazon would even notice, but it's the principle of the thing.