They can't possibly know that there are no bomb making liquids airside, they are just presuming. The alcohol in the duty-free shops comes in from bonded warehouses and its impossible to know that a bottle hasn't been tampered with. Ditto for every other bottled liquid, such as water, orange juice, etc.
Add to that all the 1000's of people working airside, and you have a hole in security that you drive a bus through.
Its a remote possibility I will agree, that a terrorist outrage could happen airside, but airside is far from completely safe.