Their lots really aren't "always full of cars". A lot of rental companies, especially franchises, have sold a lot of their cars due to reduced demand and insane used car prices.
And perhaps if you're a customer picking out Model 3 cars on the lot and you pick one not fully charged.. you could decide to wait it out or just drive.
If you don't require your customers to return a full vehicle, they don't need to deliver them full to the customer either. I experienced this in Croatia when I discovered they are actually not allowed to require the car be returned full (some local legislation apparently). So they delivered it about 3/5th full and I returned it back in the same state. They just eyeballed it and it was fine. Actually annoying like this because I had to remember when to fill it up (and by how much) in order to return it with that quantity. In the end I put about 10 liters in it when it was about 2/5th and then drove another 20km.
They'll probably do a quick top up using their own chargers while they are cleaning the car. No need for them to charge to anywhere close to 100%. It's actually bad for the battery to do that regularly. So, it's also not something they'd want their customers to do. Anything over 10% is fine and can be topped up in 20 minutes or so to about 70-80% if you have the right charging infrastructure. But probably lower once they start cutting cost a bit. I wouldn't be surprised to get a Tesla rental with 40-50% charge only. Plenty of range on that and I can fix it at the next super charger in 20 minutes. For free because its included in the rental price (also announced). They just eliminated fuel as a variable cost for the driver. They'll probably charge per mile instead.
That may just be to help avoid people planning poorly and running out of battery trying to just barely make it back to the return. There's almost certainly nothing magical about 10%, and charging the car from 5% will be almost no difference. It wouldn't surprise me if they don't charge a fee as long as they can still get it over to the cleaning bay or a parking spot with a charger.
They're likely just setting ground rules for people who have never used EVs. It's not quite as easy to solve if you run out of juice half a block away from where they want it to be returned. Or if it won't start but you got it parked in a spot out front without a charger. "It doesn't need to be full but please leave 10%" is a fair rule of thumb.
> If you don't require your customers to return a full vehicle, they don't need to deliver them full to the customer either.
Every U-Haul I've rented in the US works the way you're describing: you fill it to where they tell you. Not to a full tank.
rent the car in the morning, keep it for a while, you bring it back some evening, it gets rented out the next morning.
It's like hotel rooms. Sure you could clean them in 5 minutes after someone steps out the door, but is someone actually there to take the room afterwards?
sales dude: you want to prepay for that full tank of gas, super cheap only for you my friend, just today
me: no
sales: if it's not full it's going to cost 50% more
me: no
sales: look at the gas station prices
me: I never return with completely empty tank so whatever savings get wasted on the fuel I gift you on return
sales: but it's so much cheaper
:%s/gas/electricity/g