It's probably not just cost control--it's revenue capture. There's definitely some MBA games going on in academic publishing. Just look at the edition frequency (that's not to design a better book, it's to mess with the resale market). So when you see these sort of alternate payment models always suspect a revenue scheme.
Sometimes you have to buy (or have your school buy) membership in a online HW-solver type of club to see these extras. This is extra $$ for the publisher. And also a means of control for colleges (i.e. making it harder for self studiers. Gatekeeper stuff.
Furthermore any online structure is a hassle since it is not resalable. It's also not enduring. What if the provider goes defunct, for example? It's like the trend in "software as a service" rather than having a CD. (Same with movies from streaming services versus buying a CD.)
I'm not sure about your specific example. But I suspect some of the game with the online service (for extras or for HW) is related to edition control. For example, they can STOP having the online Xth edition content when they come out with X+1 or X+2. (see intro.)
More tenuous but still conceivable is something related to "price discrimination". https://en.wikipedia.org/wiki/Price_discrimination Marketers try to come up with schemes to separate the cost sensitive from cost insensitive and then charge different prices (often with trivial differences in the product). The point here is to compete on price only when you need to. (With a total free market, transparency, and a commodity product this is impossible. But there are many deviations from ideality. So marketers try to exploit. This is why discounting exists for instance, versus just across the board price changes like on oil, grain, etc.) I don't know exactly how they'd tie it in with a price discrimination scheme, but it's the sort of thing marketers think about.
Other than that, it's probably slightly related to the AP BC change (slow and small step backwards in coverage, "dumbing down" by cutting content). They cut 2nd order diffyQs a few years ago. (Also shells for volumes of rotation.) I think lots of colleges still have it. And it's sort of a normal part of a traditional text (Granville, old Thomas, etc.) So they probably need to keep it in for schools that haven't dumbed down yet like AP did. But AP is probably a huge market. And may be a trend that some colleges are also following, to water down.
P.s. After writing this I did a quick Google search on 'use of online content to capture revenue in textbook publishing'. Several articles came back, talking about this as a scheme for publishers to fight the used market. For example:
(Note this is a 2013 article--not a new MBA game.)
P.s.s. One of the things that makes it possible for publishers to play these games is the partnership with schools (gatekeeper effect). Obviously if you are a self studier, just buy a good copy of old style Thomas (even has all the answers in the back.) If you check Amazon reviews, you can find the editions that are praised, for example. In general, people still like many Dover texts that are 50+ years old, so recency is not very important in lower level mathematics. (Be careful and read the reviews though).