It does have a notion of function imports & exports, so in the same way it allows you to do what you want, without specifying what you're going to do. For untrusted code, like in a browser, the environment can choose only to extend limited access. For trusted code, the environment can give access to more OS-level functionality. By webasm not specifying what functions to bind, but giving the ability to bind functions, it remains flexible and appropriate for wildly varying deployments.

Basically, zero cost exceptions let you unwind and capture the stack while not slowing down the common case, and multiple entry points let you build the stack back up without compiling a large amount of duplicate code. This gives you arbitrary stack manipulation without explicit stack manipulation support in the VM. It enables features that usually require special VM features, such as GC, coroutines, lightweight threads, (delimited) continuations, effect handlers, C# style async.