What's great is you can still use React with a Meteor backend. There are tons of integrations and examples out there on how to do so. :)
Meteor is modular enough that you can swap out current pieces if you want to, but you're certainly not expected to do that (and if you're just getting started with Meteor, it's almost discouraged). And there are plenty of benefits to just using Meteor for everything -- e.g. Meteor creates a "minimongo" interface on the client that lets you re-use your MongoDB queries on the client.
One way to grok this is to compare Meteor to Django. Just as you can swap out Django's default homegrown templating system with Jinja, you can swap out Meteor's default homegrown reactive frontend with React. But you certainly don't have to and neither Django's or Meteor's devs expect you to.
There's a more thorough discussion of Meteor's various parts here: http://joshowens.me/what-is-meteor-js/
You can read more about that here: https://www.meteor.com/projects