These zones are excluded according to their blog post.

https://medium.com/strava-engineering/the-global-heatmap-now...

Aggregate data about private zones would give the company these some interesting data, especially combined with other customer data. Knowing which people don't want you tracking their location, and where, and how these zones line up between different groups of people, would give you a lot of information about what areas each group considers sensitive.

(e.g. Say you could cross-reference some of your customers with military ranks, and a bunch of people above rank X had a certain area blocked out. You could then infer that (a) there's something there that only people with that level of clearance would know about, and (b) then infer that anyone else who blocked it out had at least that level of clearance.)