Major tech companies snooping on email contents
Cyber-security company High-Tech Bridge set out to test the confidentiality of 50 of the biggest internet companies by using their systems to send a unique web address in private messages. Experts at its Geneva HQ then waited to see which companies clicked on the website.
We are most likely talking about automated link scanners, checking the links in emails for viruses designed to attack people clicking on the link, and in google's case, likely indexing the link for their search engine.
Whether that's good or bad is up to the reader to decide. I would be very surprised if an actual human is looking at those links, but having a link you send in private email to a gmail address suddenly be publicly indexed could be very surprising. If the data found at the link is associated with the people sending and receiving the email for advertising purposes, that could also be awkward.
It's one reason I don't use gmail.
On the other hand, any ISP that provides any type of spam protection -- which is pretty much all of them -- will be doing some sort of automated content analysis on your email to determine whether incoming email is spam or a message you actually want. Many of those include link checking. The difference is, Google is likely using that data for advertising, which implies storing it and associating it with your identity.
Not to mention, the admins will be able to look at the contents of your email unless there are pretty extensive technical measures taken to prevent it, and that applies to any email provider that isn't you personally. The vast majority of email providers aren't taking those measures.