Bitly links in email are super super shady.
The reason bit.ly links can end up in mails is if third parties want to track clicks separate from the sender and the sender isn't smart enough to catch it. For example, people who want to include a job listing or some sort of sponsorship or advert in your mail. They'll often use bit.ly, simply because it works fine on Twitter, without realizing it's not a great idea in mail.
* law (e.g. CAN-SPAM or the Privacy and Electronic Communications Regulations 2003) - indeed CAN-SPAM even stands for Controlling the Assault of Non-Solicited Pornography And Marketing Act and reflects the US government's position, the EU's regulations refer to 'unsolicited communications' - http://www.legislation.gov.uk/uksi/2003/2426/regulation/22/m...
* encyclopedia (say, Wikipedia - http://en.wikipedia.org/wiki/Spamming - or the Encyclopedia Britannica - http://www.britannica.com/EBchecked/topic/941678/spam)
* dictionaries (say, Websters - http://www.merriam-webster.com/dictionary/spam - or the Oxford - http://www.oxforddictionaries.com/definition/english/spam)
* technical standards (say, RFC2505 - http://tools.ietf.org/html/rfc2505)
* the viewpoint of people extremely against spam, such as Spamhaus - http://www.spamhaus.org/consumer/definition/ or SpamCop - http://www.spamcop.net/fom-serve/cache/14.html
* the terms and conditions of e-mail service providers and senders - e.g. http://mailchimp.com/legal/terms/