This is a complicated legal area. I'm a retired attorney, NY state, but specialized in tax and securities law, but did work for years in the entertainment field. Likely, I have just enough knowledge to be dangerous
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So, this is my personal opinion, not legal advice. Here are some basic guidelines as to the USA:
Generally, anytime a copyrighted work is performed or used in full or substantial part other than for comment, criticism, news reporting, teaching, research or scholarship a license is required. For example, if you are busking, a license is technically required. If a copyrighted work is used in connection with any type of commercial endeavor, say in a bar, even at an open mike event, a license is also technically required. Even if a work is used by a non-profit entity, say a school, and is to be preformed in front of an audience, a license is required.
The copyright laws provide extensive protection for authors and publishers. The allowed uses without license fall under "fair use" -Section 107 of the U.S. Copyright Law. Examples of this would be using copyrighted material in classroom teaching, for example, handing out a newspaper article to a class. Other fair use is citation and minor excerpts in research, and use of less than a substantial portion of the work in a news article. Parody is sometimes allowed, as it may be considered criticism.
The law does look at a number of factors: whether the use was for profit or not, whether the use could diminish the market value of the work, and how much of the copyrighted work was used. The intent of the use is also important. For example, if you performed a copyrighted show without license, but no one came or you lost money, it would still not be a fair use.
Now, the law is relatively clear that many of the sites that publish copyrighted works, even if they are "original" interpretations, are technically in violation of the copyright laws in the US.
Practically, however, most copyright holders are not equipped to, and do not choose to, chase down every use of their works. As noted above, some will send a letter to a venue and hope that the use stops. Most of us, me included, play a lot of copyrighted material without specific license. If for personal use, no profit motive, I feel quite comfortable using the copyrighted material. And, I also do so when I play for kids and others. But, I do not charge anything. I also use copyrighted material with my students, family, friends and the like. And, a good part of what I use is based upon music books that have obtained licenses to publish the works. For example, any of the material from respected publishers like Hal Leonard, are legal, in that they obtained the necessary permissions to publish their books.
The sites that put up "for educational purposes only" are trying to shoehorn themselves into the fair use doctrine. Whether they actually fall under that or not is a question of fact and law. Some may, others may not. It is unlikely that anyone will chase most of them, it is just not practicable.
Here are some simple guidelines I use to govern my own behavior: I obtain almost all of my material from a licensed source, such as the
Daily Ukulele. Some of it I just noodle out on my own. I do not perform for profit and do it just for fun. I do share my take on songs with others, but I do not publish tabs of them. My take on ukulele club jams is, if you are teaching each other and you have a licensed copy that you share it is likely a fair educational use, teaching need not only be in a school or university. Jams at a ukulele festival, where admission is charged, are likely a different matter. And, it is important to remember that no one cannot copyright a C chord in and of itself