Just because it's illegal, doesn't mean health insurance companies don't find loopholes, and consider fines when they get caught as the cost of doing business. See this series of articles[1] for some of their criminal shenanigans.

It's more than likely that they would use genetic data to deny insurance, and then settle the cases in court if they happen to get sued, which statistically is probably a rare occurrence.

[1]: https://www.propublica.org/series/uncovered