They do this as a benefit for the religious public that won't be using the car on that day anyway. If I had to pay for the rental on Shabbos when I was not using it, I would likely not take it for Friday or Sunday, so they benefit by having me rent the car for the period of time I need it, and I benefit by not paying for a day I am definitely not going to use it. I don't think they have to do this, but it is very commonly done, and it seems to be a good business practice.

What happens if I rent the car stating I am Shabbos observant and then use the car on Shabbos? I don't know. I don't know if they have a way of knowing, and I don't know if there is some penalty if they find out.

It seems there is a similar, but different, clause in the contract one signs when subscribing to a plan for a kosher phone in Israel. The contract says that the user is not allowed to use the phone on Shabbos. If the phone is used on Shabbos, the charges will be exorbitant, at the rate of 17nis per minute. That is a ridiculously high rate for using a phone, but it does not bother most (all?) people getting the kosher phone because they don't use the phone on Shabbos.

One fellow had to make a phone call on Shabbos. It seems he knew it would be likely that he would need to make the call, as he sent the provider, Rami Levi Communications, a notice in advance asking that they waive the charge as he would possibly need to call a doctor for a medical emergency. He received no response agreeing to waiving the fee.

On Shabbos the situation arose that he needed to call the doctor - his son had had a surgery and was in pain and they needed to call the doctor to discuss the possible need of returning to the hospital. He then saw at some point that he was indeed charged the higher rate for the Shabbos phone call. He then wrote another letter to the provider complaining about the charge. He explained the medical emergency and that he was charged the 17nis per minute rate despite that he had asked in advance that it be waived. He admits it is just 17nis per minute (just???), but in this instance it isn't reasonable because of the medical emergency and it is shameful for them to collect it.

The writer accuses the company of not having a "jewish heart" and that obviously he only needed to use the phone because of pikuach nefesh and complains that it is them taking advantage of the situation.

Rami Levi Communications response was simply that the use and charge was exactly as per stipulated in the contract, there is no cause for issuing a credit.

source: Hamechadesh

I feel bad for the guy, as it was an emergency, but I never like the complaints people have about being discriminated against because they choose to use a kosher phone and cannot take advantage of certain deals or technological benefits offered to smartphone users. They choose to forgo that and use a kosher phone, and that is the price to pay - it has other benefits that they prefer. I think accusing them of not having a Jewish heart and being cruel is a bit over the top. He tried to get it waived, and that is fair, but they did exactly what he signed up for, and then he got unreasonably upset at them.

In my line of work I see many people who install various filters on their internet for their computers and phones, and then want to find ways around the filters to use some services that the filter blocks. I always tell them that if they need the filter, this is the price to pay. It is their decision to use the filter or not, but you have to understand that the filter is doing its job exactly as you wanted it to do, and cannot get around it. If you don't want the filter, and want instead to use this or that website or service, get rid of the filter.

if you want to have a phone that doesn't cost so much for Shabbos calls, get a  landline or a different phone and not a kosher phone that tells you they are charging extra for Shabbos use.