Why not? Hosting is cheap and my goal isn't to maximize revenue, it's to maximize personal gratification. I get the latter from free readers as well as paid ones.
Maybe the free version cannibalizes sales, but I'm certain the book is much higher profile and reached a much larger number of potential customers because I gave (and continue to give) the web version away for free.
I would argue that providing the book content for free as a website will sell more books vs only having the for-sale version.
Especially with a niche like this, until you're an established author that people will just buy because it was written by you, people will require some sort of evidence that you know what you're talking about. Is the book worth it? Reviews are good. Better? being able to skim or read the book before buying.
Due to how temporary websites are - I think the physical copies and the ebooks are a sound purchase for people that will want to be refer to this book at some point in the future.
Maybe the free version cannibalizes sales, but I'm certain the book is much higher profile and reached a much larger number of potential customers because I gave (and continue to give) the web version away for free.