Apple has a pattern of purchasing companies that already offer products on other platforms and immediately shutting all non-iOS products down.
Obviously, they're free to run their business however they want, but it's also understandable that fans of those products might be upset that their app (or APIs that power other apps they enjoy) are no longer available simply because Apple did "their usual thing". Many companies (including Apple in rare cases) maintain or even develop codebases for applications across more than one platform.
There's nothing wrong with Apple buying companies to shut them down. It just sucks for the end-users who no longer get to use those companies' products for the sole reason of "they got bought by Apple".
People are raising concerns over Apple buying a small company and shutting down its offerings to any platform that isn't Apple's.