I'm sorry this happened. Definitely keep bringing your bike in at night.
But, I'm honestly surprised that anybody would leave their bike out. You live in the city and while it's not Compton it's still a city. Don't look at any area as being "friendly". A lot of crime is hit or miss. You may have had a good experience living in Cambridge while someone else on your street might not have had such a good experience. I can't stress this stuff enough here. Protect yourself and protect your belongings. Always be cautious. It's better to be safe then sorry.