I'm not sure why but some people like to assume that just because you posted something online that you must fix any problem people have while using it and add any feature that it doesn't have. I suspect that this is the main reason those "This is a hobby. Please don't get mad at me if there's a problem" README tags started showing up.
Of course, there's no obligation or even expectation that you should provide such a warning, but if you're already thinking about it, might as well just do it.
It's not realistic to read through all the source of every dependency you use, but it should be encouraged for developers to take a more active approach in evaluating dependencies they pull in - at least to check how active development is, what sort of open issues there may be, test coverage, code quality sniff tests, what the license is etc.
I'd be willing to bet most GPL violations are due to ignorance, rather than intentional.