If a venue charged referees then a percentage of referees would only accept games for venues that didn't charge. There is usually a shortage of referees to begin with. The leagues/tournaments would wind up paying more in referee fees because the Associations would reimburse the referees the parking fees (they do that at Silverlakes). The additional cost to run the tournament or league would just be passed on to the clubs that pass it on to the parents.
Large Venues (Silverlakes, San Bernardino, Galway Downs, Oceanside, SD Polo Fields) want attract the large tournaments. Making it easy for the customer (Cal South, SCDSL, Coast, Albion, Arsenal, Surf, etc.) means, making it easy for their necessary partners (Referee Associations).
In my capacity as a parent, I appreciate that parking fees are a necessary evil. I have seen first hand the cost to maintain 40 acres of grass. These venues will incur anywhere between $400k to $800k in field maintenance costs over the course of a year (water, fertilizer, aeration, damage remediation, sand, weeding, gopher control and labor). Silverlakes is on a whole different program with overhead costs estimated to be $3M, of which its likely nearly $1M is related to maintaining the grass.
Somebody has to pay this and congratulations its you.