I know why it happened, but it still strikes me as odd, the fact that the goalposts kept moving, as it were, with copyright. And it’s weird no matter whether the copyright is there to give other people rights to use, copy and modify the work, or rights to the author to protect and profit from their work. In other areas of the law, the general rule is that what counts is the law at the time. It’s only illegal if it was illegal at the time the offence was committed, for example (the major exception being crimes against humanity). Even patents are valid for a set period of time, and companies know how long that will be when they apply for the patent (hence all the pharmaceutical tricks with minor modifications that they hope will be just enough to get a new patent on). Only in copyright, that I’m aware of, has it been the case that the period of validity has been so massively changed and applied retroactively. From 21 years (see History of Copyright to the death of the author plus 50–75 years, depending on the country you live in and some convoluted dependencies. And then there’s the famous extension by which Mickey Mouse would have been in the public domain by now, but won’t be for a while yet.
It just seems odd to me, the fact that copyright is the exception to the general rule. But maybe it just seems odd to me.