It's just with the default browser and yes, it's a main source of funding. This is what they say about it:

  Search engines help fund Linux Mint: Search engines share with Linux Mint the revenue generated for them by Linux Mint users. The following search engines share revenue with us: Yahoo!, DuckDuckGo and Amazon. By using these engines for search or to make purchases, you help fund the Linux Mint project.

  Easy installation of other engines: Clear explanations and easy instructions are there in Linux Mint for users to understand why some engines aren't installed by default and how to easily install them.

from: http://linuxmint.com/rel_maya_whatsnew.php

e: Clarification

It is a source of funding, which would be fine with me, but it was also basically unusable, and it would give me weird results that I didn't get on regular Google. I also had a bit of trouble removing it. So bad experience for me overall.

Edit: no, they weren't so sinister as to install it with another package, it's part of the Firefox extensions in the default install.