There are two sources for these numbers, maybe even three.
The first one is looking at the maximum fines available for disability-related violations under the current legislation, which is pegged at $250,000. It is not going the full $250,000 per offence, but it is still significant.
The other source is looking at the airline's revenue. If the fine is too small, it's just a cost of doing business.
The third aspect is one on which you can do a far more precise analysis. It's what I colloquially call the Kaplow formula. It is based on a textbook on analysis and law where you use probabilistic methods to gauge what kind of fine is necessary to dissuade a particular conduct, bearing in mind the potential for financial gain from that conduct and financial gain from preventing that conduct, how much it will cost the airline to take steps to avoid that particular conduct, and how often that issue gets detected. There is a science to it.