The idea is to be able (whether you see a use case or not) to use
different tasks lists simultaneously.  Messing with globals is the worst
possible API for that.  All you need is to add a tasks=None argument to
the loop() signature, rename the global tasks list to (e.g.)
default_tasks, and add this to the top of loop:

if tasks is None:
    tasks = default_tasks

similar to what it does for map.  You'd also have to pass the tasks list
to the scheduler() call and the call_later() constructor.  Defaulting to
a global is fine.