PaHunter, I think the "official" vendors get information shared with them earlier in the day such as "we have PaHunter assigned to site 1108 so go ahead and set them up at 3pm". Maybe WDW has a deal with the officials to clean/prep their sites first and make them ready (I would for what the officials pay.
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The non-officials have to wait until the guest, PaHunter, gets the text or notification of their site being available (could be 5pm) and THEY have to send that info to the vendor to allow them to set up while the guests wait. Being an "unofficial" vendor makes it a little more inconvenient for the guest (but less expensive for the guest as well). The vendor knows the policy under which they can deliver the trailer and set it up and communicates that to the guest well in advance.
Disney 'don't do nothing for free any more if they can help it' (said in my best Queen's English). Paying Disney for the privilege of being an official WDW vendor means you have to have some differentiation from the non-official competition. Or, put another way, they "inconvenience" the competition to provide a competitive advantage for which the vendor ends up charging a higher fee to the guest and then passing on some $$$ to "The Man".
Bama Ed
PS - it's like at college football games, they started selling "Stadium Radios" for $20 a season to fans attending games to hear the radio broadcast in real time. But, you might say, ANY fan can bring a radio to the game and listen in (near) real time. Oh no, my friend, when they introduced the Stadium Radios, they also put in a 7-10 second delay on the public/regular radio broadcast per a rights contract to "encourge" fans to pay up (for which the University gets a cut of course). As a season ticket holder from the day I graduated, I HAVE NEVER PAID FOR A STADIUM RADIO. And after a decade+, now it's only 3-4 seconds of delay (still not paying).
Bottom line, they will take anyone's money but they will charge more (through the vendor) for getting the site sooner. No trailer vendor has ever been turned away that I've heard of.
PPS - anyway, that's what they do in my college conference. Don't know what they do in other minor conferences like the Sunbelt, SWAC, and Big10....
PPPS - they inconvenience the golf cart only vendors the same way. If I were a guest using these services, I would simply leave my arrival day open to take care of getting in and getting set up (with maybe an evening dinner ADR over at Raglan Road in Disney Springs).