Why? You paid for the software to run in a PS4. and it runs. And it will run on a PS4 pro as well. It won't have the enhancement that is possible in the pro because it wasn't coded in the software. So if it required for the devs to revisit old code and write who knows how much more new code, new assets etc to make it better (4k upscale/HDR) asking for a fee is ok on my book.
I write code for a living. Why in the hell do you want me to spend how much more time in a old game that works fine for free?
I don't disagree, but presuming that the Pro sells well wouldn't there be an argument that making an improved version for that platform could lead to new sales?
I don't imagine that many 2013 games sell all that well 2-3 years later, but if there is a new shiny coat of paint applied it might lead to a healthy bump in sales for those who skipped vanilla PS4 and went straight for the Pro.