Earlier quoted context omitted.
Actually I'd argue that you're more obligated to abide by something you explicitly checked "I have read and accepted..." than something that was listed somewhere in a file you downloaded with some other stuff. You probably just happen to like the terms of the GPL or ASL more than you like the terms of Google's API. I do too - but at what point are we just losing all integrity because we're too lazy to read a long doc…
If there is an actual click-through and the license is understandable by a human , perhaps. Usually there is neither, though. I rarely even know how to look up terms of an API, or that there is an API. If I know the name of the API, I can look for "Google Foo API", but that name isn't always obvious. If I just see a URL with an obvious schema, and I change the HTTP GET parameters to retrieve different stuff, am I all…
Playing dumb isn't an excuse. Can I stop paying my morgtage on my house because the 40 pages of documents I signed I don't 100% understand? No.
If you want to enter a legally binding contract that you don't understand you hire someone to help you do so, or face the consequences if you signed something you were not smart enough to be signing.