If you are a member of a gym, or a working men's club and fell out with the other members, cancelled your membership and stopped paying your subs, and then turned round and said you still wanted to use their facilities, the subsidised bar, they'd think you were barmy. It will be the same with the EU.If you joined a gym for £50 a month, used it twice a month but could cancel your membership and pay as you go for £10 a time why would you carry on paying £50 a month? And why would the gym refuse to let you pay £10 for your occasional visit even though anyone can walk in off the street and do just that?
To stretch Mr Dobson's analogy a bit further, the gym would generously give you back £10 of your membership fee which you'd get by way of a voucher every month that you can only spend in their shop where they'd tell you what you had to buy and then you'd have to wear a t-shirt with their logo on thanking them for giving you £10. If you told someone you were going to join a gym on those terms they'd think you were barmy!