What you do with the software is up to you.
There is also something about it in the FAQ
# I am having problems with the admin at a certain board, help!
# A board has ripped off my graphics/software/etc., stop them!
# A board is dealing in warez/porn/etc., you need to prevent them doing this!
We provide the software, we have absolutely nothing to do with any board that runs it (beyond phpbb.com of course!). The GPL grants the user an unlimited right of use subject to their adherence of that licence. Therefore we cannot prevent, dictate, control or otherwise limit the use of phpBB 2 software. So please do not contact us for such matters.
If you have a problem with a given board please take it up with them, not us. We are not and cannot be held legally responsible for any third party use of this software (much like Microsoft et al cannot be held responsible for the use of Windows in illegal activities, etc.). Additionally we do not track the use of phpBB software in any way. So please do not ask us for details on a "given" board we will not be able to help you. If any law firms or lawyers out there send us writs, Cease and Desist orders, etc. for third party website use of this software we reserve the right to charge for time wasted dealing with such issues ...
Also, when you say "phpBB doesnt allow"
Please dont confuse us to the various phpBB hosting services wich usually have these types of restrictions in their terms of service.
Those sites have absolutely nothing to do with phpbb.com and we are no way affiliated with them.
Again, they just use our program.