Because we are clearly just floating along in the zeitgeist and completely, if subconsciously, attuned to the academic universe, it turns out this question arose the other day on Twitter. Andrew Kern started with this tweet and then continued to dig (full Storify of the tweets here).
The answer, it turns out, is that the very large amount of money that PLOS makes goes into all kinds of surprising things; huge (but unfortunately normal) CEO salaries, investments in stocks etc, building up reserves, and investment in the company and it's infrastructure (in particular a new submission system). Michael Eisen then came back with some useful context (full Storify here) which addresses some of these issues; much of the investment in the company is around open access advocacy, etc).So I was staring at an invoice for page charges at PLoS Genetics ($2250) and wondered what could they be doing with all that money 1/40— Andrew Kern (@pastramimachine) March 15, 2016
PLOS created open access, and proved it could be done and make money. This is a remarkable achievement done in a pretty small amount of time. They are strong advocates for open access and this advocacy requires time, people and money to advertise, attend conferences, lobby and more. All of this is important work and I really do appreciate it - open access simply must be the future of scientific publishing.
I am, however, still not convinced that I should be paying for that activity via my article processing fees.