Uhm, the whole point of services like these is that they are external and not tied into your own hosting.
Also, it knows absolutely nothing about your services and doesn't require any level of access. All you do is ping it.
It requires internet-level access. So internal hosts that shouldn't have internet access (like a backup machine, where all your most critical data is stored) might not be able to reach it.
So, in case you didn't notice. This can be a way to let others take care of notifying you when cron's fail so you can focus on more productive tasks.
I can set it up for you on your *ix machine in an afternoon for $200 :)