I'm a monthly parker next to the fabulous Fox Theater. When there is an event, the price to park usually goes up for daily parkers and I stand a good chance of finding a parking spot. But sometimes management does not anticipate an even and does not raise the daily rate. When that happens to lot fills up quickly, and I have no place to park in spite of have paid for monthly parking. I have learned the hard way that the parking price is really important to making sure that there are available parking spots. The daily parking price makes my monthly pass more valuable.
So why does this matter to Tesla reservation holders? What is the value of a reservation? It simply holds your place in line against against others who might crowd you out. If Tesla were to offer free reservations to some, but not others, it would devalue reservations for anyone considering one. If those how came before you were given free reservations, then the line is already longer than it would have been had all reservation holders had to pay. The longer the line is ahead of you, the less value your reservation holds. Moreover, you might be inclined to think that if other people got free reservations, perhaps you deserve a free reservation too. The thing that makes the reservation worth the deposit is that you know that the deposit is required of all other reservation holders. If the reservation is not worth the deposit, then you wait and allow all others who are willing to make the deposit to jump a head of you.
At some point you may see that your competitors are getting rigs that give them a 25c/mile cost advantage over you, you may appreciate that getting a reservation is worth the cost. So when Tesla does start building Semis, the value of a reservation could go back up as to back log grows. Tesla will want to be able to command a $20k deposit on reservations at that time too.
I suspect Musk understands this quite well and would not countenance a policy that would undermine the value of reservation. Sorry, bears, there's no such thing as free parking next to the Fox Theater.