Neither the law nor the RIAA say that copies cannot be made. They are saying that copies should only be made by those who are legally authorized to make them. In other words it is a legal monopoly, not an outright ban.

In theory if there exists a large enough demand for this music in written form, then it will be worth people's while to provide that. For personal reasons some musicians may not want to cooperate, but there is still a large body of legal written work available to learn from. Perhaps the music that you want to learn is not there, but other music is.

Heck, I am not a musician but I happen to own several books of legally published music. It is easy to find other books from a variety of genres. I'm sure that your employer would be able to sell you some. So it is not just in theory that this legal market exists. As I said, not everything that you want will be available. But it is available, at an inconvenient but doable cost.

Whether you agree with this law or not is a different issue. Plenty of people disagree with copyright and it has always been widely violated. (In fact there is even a legal doctrine on when it is OK to violate copyright.) My point is that under the law, the RIAA is clearly in the right here. And it isn't "under the law because they got an absurd bill passed." It is a pretty basic and well-established part of the law.

Cheers,
Ben