Travel agent didn't fleece you - they offered a service & choice to you. They can't force you to book them.
Look at the flip-side - what if you weren't offered the service, got checked in and discovered you were sitting in different bits of the cabin, then hear from other pax that their travel agent offered to prebook their seats for a tenner each - how peeved with your agent would you be then?
I am an agent and would much rather a hundred punters laugh at me & tell me it's a rip off to pay a tenner each for sitting together and decline my kind offer, than have one coming in after their jollies to have a winge about not being offered the service in the first place.
I personally have been at check-in at ETD -4 hours for a CUN charter which didn't have the option of prebooking and still the 2 of us couldn't get sat together.......we weren't even in the same cabin!
Finally - it's not just First Choice (tourop) doing this on packages which have charter flights.
You will find that JMC, Cosmos, Panorama, Manos, Airtours, Thomson, Olympic, Kosmar, Libra to name a few all charge AT LEAST £10 each for sitting together. I'm sure one of them charges £15.
cheers
FF