Interested why I am doing that? Because I like to reuse my own pictures without asking the publisher for permission first. And I want that you can do the same. I think no sketch, unless it is clearly a piece of art, and no plot of physical data whatsoever deserves copyright. Only Nature has its copyright!

Worried that you did something wrong when using a picture without asking for permission? Well, in most cases you will be fine if you use it for a talk or thesis as you have no financial interest in it. But it's a gray zone. And at the latest when you want to use another picture in a scientific publication, in particular a book, you will experience that it is quite cumbersome to ask every time for permission.