What's hidden about them? hidden is adding something onto your bill you don't know about, a voluntary fee is something that is added/removed by you.
So many times I hear people moaning about FR, how they paid £160 for their family of four to have their boarding passes reprinted at the airport, well if you will totally disregard the advice that the airline urges you to read (FR make it VERY clear), then it's your own stupid fault. Same with baggage, if you get to the gate and your bag is bigger than the dimensions, shut up and pay the fee; you had the rules in front of you.
I've never paid any airline fees, except a dose of priority boarding every so often, because I do what the tin tells me, stick to the rules and a happy traveller you shall be.
The upside, is the ignorant, incompetents subsidise my cheap fares.