‘Por’ after pagar or not?

Hola J.H

Generally, if what we want to say is simply "paying for something" as in making a payment, like the example about the beers "Deja que él pague las cervezas"; we don't use the preposition "por", as the example is showing.

If we add "por" (pagar por algo), like in that other example: it could have an emphasis on someone paying for their own things and for others as well, as in being very generous and inviting everyone. Although this sentence could also be interpreted differently because "pagar por algo" can also mean "to suffer the consequences of something", so, we could see a different meaning in "No voy a aceptar que pagues por todo" = "I'm not going to accept that you take on the entire burden, whether financially or in terms of responsibility or consequences." 

I hope this clarified it.

Saludos