I don't believe it's at all inappropriate to ask for their feedback. I literally do this all the time (in causal inference, my field, it never hurts to reach out to subject matter experts). In my first paper (currently RR'd at Stata Journal), I thanked A LOT of people in the footnotes, because A LOT of econometricians took time to read my work and give very very good feedback on it... but I didn't include them as coauthors to that paper because that paper is mine, and mine alone.
With this said, if they want to be a coauthor cool, that's great. But, I do believe it is wrong to ask them for co-authorship, unless and until you're prepared to work on it as a proper collaboration (that is, a true collaboration, not simply feedback). In this business, authorship is warranted with contribution. If you did all the hard work, if you were up til and past midnight running code to solve problems and make sure things worked properly, if you've revised the manuscript 100 times from beginning to end, taking every pain to ensure that it's written as logically and as succinctly as possible, if you were the one who called all the shots and had eureka moments at 2am about the way to solve the newest problem you'd been facing, then you are the one who deserves authorship, and while other people may give comments (that we are very most grateful for!!), they should decidedly not be included as co-authors, unless they are prepared to make it their paper as much as it is theirs, and vice versa.
Authorship is conferred with contribution, not simply strength or knowledge.