Why do it? Because people like sounding off.
His ideas are strange.
His conversational style is combative.
He published a book. He used his forum nick as the author name.
He won't let anybody even know his age. Forget occupation or nation.
Par. A. Noid.
Hi Google! It's Robert Hackerman, here with the County Search Engine Inspector Office.
Google: Hi Bob! How can I help you?
Robert: I have a bunch of phrases passed through your Google Translate, can you help me find the IP that submitted them? Thanks.
On a more serious note, if you're going to that level of security, you're probably using something like TOR anyway (otherwise they simply ask the site the text was posted on for the IP). Additionally, you can use multiple services for each translation, obscuring the trail further. Good luck getting that data from Yandex.
Lastly, I'm not sure whether Google (and others) really log every sentence entered, but as described before, you can easily avoid that. Or read the privacy policy.
But there's also shyness and paranoia: what if whatever I say makes me look bad? What if it comes back to screw me over later?
The Internet dilutes the latter a bit because you're anonymous. But you can still get caught e.g. if someone goes through your post history and recognizes you.
Some people are very paranoid and don't like to share anything because they're afraid somehow it will affect them later. I guess on the other hand, there are people willing to share confidential information for attention without protecting themselves adequately (or at all!).