While I agree with Magiver about acting in accordance with the expected rules of where you’re at, I have to say in regards to libraries, that isn’t the best argument.
Most libraries don’t have rules specifying quiet any more, and several have rules specifically ALLOWING various levels of noise now.
Libraries are no longer places where “common courtesy” expects people to be quiet. They’ve gone from being like churches or cemetaries to more like mall food courts or town squares. I’m sorry that you don’t like it, and I really do mean that - I am a librarian, and I like to help people and make them happy. But the majority of our patrons like it better this way than when we were quiet and strict. 
I think that a lot of people (especially older people, or academically-minded people) are expecting academic-library atmospheres from public libraries, and that’s never going to happen again. It USED to be true because the majority of patrons came in, got a book, read a book, read the newspaper, and left.
The majority of our users NOW come here to use the internet, to have a somewhat quiet (haha) and professional place to use their phones, laptops, and faxes to conduct their business, and to attend events - usually centered around children, but including book discussion groups, quilting bees, and other non-book related activities. Sure, people still come in to look for books, but even that involves catalog computers and noisy typing on a keyboard!
When people are passing in and out of our meeting rooms (which aren’t soundproof anyway) or are filling up our computer stations (which are in the middle of the library) or are congregating for a summer children’s program, then things are going to get a little loud.
I actually prefer it that way. When we’re humming with activity, we can see people using and enjoying our various services, and we feel appreciated and part of the community - those patrons who need and use us are going to fight for us to stay open and stay in their towns and community centers. If we’re dead as a doornail and quiet as a morgue, then we don’t have a leg to stand on when budget cuts come around every year.