Basic law of contract, if you are not happy with the terms offered, refuse to enter into the contract and walk away.
Yes venues can charge more for a wedding as often the consumer will pay it. Should they? Well that depends, keep in mind many venues will make 'extra effort' for a wedding by cleaning up the gardens, supplying more staff, ordering more flower arrangements, setting up the venue differently and just more time and effort in general. Others simply charge a wedding tax.
Its completely up to you, sounds like you are not happy with the price, in which case look around and find some terms you are happy with