Provided you check in on line and allow the system to give you a seat, there is no check in charge. So it isn't compulsory.
It used to be. I remember they charged for both on-line and airport check-in but charged a lot more for airport check-in. Plus they also increased the check-in fee if you added a bag which I thought was very scummy!
They still do this with airport check-in. For example on EMA - DLM in August it charges £49 return for the bag but increases the airport check-in fee by £6 for every bag so strictly speaking it actually costs £55 per bag (and I have seen even more severe cases than that).
I never considered flying with them because of this but at least you can now avoid it by checking in on-line. Glad it has somewhat changed.