[jwilk]

Some do, some don't.

The BSD licence that MINIX 3 uses actually requires giving credit:

Copyright (c) 1987, 1997, 2006, Vrije Universiteit, Amsterdam, The Netherlands All rights reserved. Redistribution and use of the MINIX 3 operating system in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

[...]

Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

[...]

Source: https://github.com/Stichting-MINIX-Research-Foundation/minix...

[TheCoelacanth]

Exactly, a lot of people think of the BSD license as not reserving any rights, but it actually does reserve a few rights, such as the preservation of the copyright notice. A license that actually reserves no rights would be something like CC0.

[drewbug]

Or the WTFPL ;)