It's strange -- I would expect debit cards to be less risky because they're backed by actual funds from a bank account, rather than credit cards where someone can not pay/default on the loan.
This can be solved for debit cards by charging at the start of the rental, but what if the renter damages the car (or doesn't fuel up, or does anything else that needs additional charge), and again the charge is refused? The debit again is much more dangerous for the rental company here - CC can be charged (provided the limit hasn't been used up) much easier. True, people can default on CC payments - but it's not the rental company problem, it's the bank's problem. That's why they charge the outrageous interest on cards (20%+).
I guess the rental companies that fuss about CCs has some kind of kickback.
I’ve even made a $25,000 purchase that I was reimbursed for.
Plus, then if you can't pay, it's visa's problem, not theirs.
The amount can vary depending on the rental company, the type of car and - obviously - the location and is usually an amount between 1'000 and 3'000 Euro.
It's usually the amount of the deductible if you break the car.
Some hotels will do that too.
The amount is released after you return the car, but during the time of rental it's deducted from your card limit.
E: clarification
Credit cards allow them to get a lot more money after the fact than a debit card AFAIK in case you damage the car.