It is amazing they are adopting such a high handed approach in the present job climate. You can bet your bottom dollar they are desperate to get your bum on their seat...
Don't be emotive about this but definitely DON'T sign it - instead write to them highlighting the problem area. Keep your argument polite but be assertive! Tell them you will honour any reasonable contract but that you cannot accept financial liability for events outside your own control.
Some of you young guys and gals probably don't realise this yet but the tables are slowly turning in the aviation industry. You don't need to be treated like this any longer - use your integrity and brains to shame the airlines into giving you a sensible and reasonable job offer in the first place.