ba-ba.... yes it is true that some races certainly do.
I did at least 5 WFRA races last year and only recall a surcharge on one of them. So i don't think it's a compulsory ruling. I can say that it certainly worked... the surcharge was an extra £7 on top of a £3 entry free, and i made sure to join them this year!
You also need to be in the WFRA to feature in the Welsh Champs and North South/Wales Series. If not, you are scored in the individual races but part way into the season they do a champs table and you suddenly find yourself off the table!
This is however clearly stated on their website.