I feel that it's important enough to reiterate that PLOS is a non-profit organisation and relies on donations to keep it going:
http://www.plos.org/support-us/individual-membership/#make-a...
http://www.plos.org/support-us/individual-membership/#make-a...
The donations and open-access merchandise are a nice extra.
They are turning fabulous profits (almost $3 million in 2010; $4 million in 2011 -- these are net profits), but, being a non-profit, must invest them back in the organization.