What is the purpose of international editions of books sometimes omitting some problems, footnotes, or other content? I've noticed that - at least in the US - a book is published and sold at a non-trivial price to US residents. However, the author/publisher may also make an international version.

This version is subpar in my opinion. For example, they are cheaper, but also typically paperback, and I've seen in a science textbook things such as "this problem has intentionally been omitted for this edition" and "this [foot]note has been intentionally omitted for this edition."

Why are international editions stripped-down versions of their original counterparts? Why the need to omit certain problems and footnotes?

(I had asked this question first in Law Stack Exchange, but they deleted it and suggested I ask it here. So, any help on the legality of these reasonings would be appreciated).
In some cases, these international editions are sold at a fairly steep discount compared to their U.S. versions. This is intended to increase their accessibility to students in developing countries.

The intention behind omitting material that is in the "nice, but not essential" category, such as a particular problem etc. is to interfere with its utility in the U.S. and other "expensive" markets. Basically, it's to prevent the international versions from being marked up but still sold below the cost of the U.S. version.

As for the legality...the publisher owns the content. They can do pretty much whatever they please with it.