MPLS Common Practices

Have a few questions regarding MPLS infrastructure and encrypting traffic:

1. When an enterprise is using MPLS, is it safe to assume they are using a service provider's infrastructure for such and not running their own? (enterprise the size of a university)
2. I'm under the impression that when using a service provider's MPLS infrastructure, the traffic traversing it is essentially going 'over the internet' and thus you should encrypt it. Is this accurate?
   1. If yes, then why does one not encrypt traffic sent over a non-MPLS WAN link?

I hope these questions make sense. I appreciate references to read up on as well. Thanks!