This could in a sense be viewed as a form of bundling (https://en.wikipedia.org/wiki/Bundling_(antitrust_law)) where the services being bundled are the OS and the weather app.

The problem is that bundling is only considered problematic if your product is in a monopoly position. Bundling Internet Explorer was problematic for Windows because Windows was considered to have a monopoly in the OS market.

That depends on the interpretation, which differs by jurisdiction.

The EU, for example, aims its anti-competitive regulation towards firms who have a "dominant market position" rather than a "monopoly".

What competition? Is it “monopolistic” every time that any company buys a smaller company instead of developing it in house?

I don't think anyone's raising any problems about Apple buying a small company instead of developing comparable functionality in-house.

People are raising concerns over Apple buying a small company and shutting down its offerings to any platform that isn't Apple's.

So Apple should be forced to develop for a competitors platform?

Nobody's saying that either.

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".