If they didn't want off-network users to use it, they would firewall it to just their subnets. I get they have a very large network that is ever expanding, and it may just be easier to not lock it to their subnets, but seriously it's not that hard.
I don't use my ISPs DNS because they resolve non-existent zones to some bullshit landing page in which they try to "help" users find what they were looking for, effectively breaking DNS in my opinion.
I don't use Google's because it sucked the last time I used it (when it was new, I suppose it is probably better now). Tracking isn't a real concern of mine in terms of DNS, although I do block Google Analytics via dnsmasq on my router. I just don't trust Google. They abandon services all the time. Quite frankly, I didn't expect their resolvers to stick around this long.
I own a web hosting business. We have a few servers in a datacenter. I run my own resolvers that are locked down to my /25 subnet, they resolve off the roots, specifically d.root-servers.net, and e.root-servers.net. Get less than 2ms on those.
At home, however, Level3 is still faster than any of the roots. :-/