What do you mean “slip in”?
Are you saying that the term wasn’t there when you signed the contract and the added it later? If so, no they can’t.
However, what I think you’re saying is that it was there all along, you didn’t read it, but you signed it anyway? If so, yes they can.
You were told this was a term of the contract (not reading it is on you, not them) and it’s not an illegal term so, when you signed it, you agreed that they could act as your agent with respect to admitting liability and paying the fine.
The only way this term would be disqualified is if it was unconscionable (under common law) or unfair (in jurisdictions where consumer law raises the bar). This is neither. It requires the consumer to pay the fines that they incur while using the company’s car - which is self-evidently fair. And to waive their rights to defend those which, in the context of the balance between the relatively small number of customers who incur automated fines (if you get pulled over and ticketed, the issue doesn’t arise), the relatively few who would contest them and the vanishingly small number who would be successful and the administrative burden this places on the hirer verses the value of the use of the car - this is definitely not unconscionable and is probably fair. Of course, you have the right to take the car company to court and make your argument that the term is unconscionable or unfair (if your jurisdiction supports this).
Not only is this legal, it is standard practice with every hire car company for fines that don’t require naming the driver. Some jurisdictions have a points system on the driver’s licence, in those the owner of the vehicle names the driver and the state issues a new ticket to them. However, even in those jurisdictions, fines for parking offences are paid by the hirer and charged to the hireree.
In a comment, you say that the terms make you “responsible” for fines. Responsible means the same as being responsible for damage to the vehicle. It means you will pay for it, not that you will argue about it. Responsibility in law does not necessarily mean that a person has the freedom to act as they want - freedom can be traded away by contract, indeed that’s what a contract fundamentally is - you voluntarily limit your freedom of action in return for something from someone else.