How can P2Pool charge a fee?
Since November 17, 2011 no fee is charged.  Instead an optional donation of 0.5% is sent to the author of P2Pool to support future development.

If you don't want to donate to the author you can run P2Pool like this:

python2 run_p2pool.py --give-author 0.0 [USERNAME] [PASSWORD]


The 0.0 is a percentage of your earnings and can be set to whatever percentage you like.