This seems less than ideal to me.

1. Different languages have totally different allocation requirements, and only the compiler knows what type of allocator works best (e.g. generational bump allocator for functional languages, classic malloc style allocator for C-style languages).

2. This perhaps makes wasm less suitable for usage on embedded targets.

The best argument I can make for this is that they're trying to emulate the way that libc is usually available and provides a default malloc() impl, but honestly that feels quite weak.