I travel quite a lot, both in my professional and personal life, and I've made an interesting discovery:
Some busses and trains that have free wifi on board, block access to 126.96.36.199 and 188.8.131.52!
Some public services, and especially services that are pay-for-access, (like truck stop wifi) or any wifi that has a "landing page" you have to either click I Agree on or enter the daily code on the bottom of your receipt or whatever for, (or any place that uses the same setup even if it IS free) will block access to other DNS servers.
They do this because that's how their landing page works. You DHCP and get THEIR dns server. When you try to go to google.com or whatever, their DNS doesn't see you in the "I clicked Agree" list, and so it gives you their landing page IP address for ANYTHING you try to look up. All roads lead to the landing page. Only after you've clicked I Agree does their DNS work normally. So naturally they need to block other DNS servers or you'd just walk right around their landing page. It's just easier to redirect DNS than to try some other way to block you by some other method.