Sorry for the late reply, But I could not stop my self from answering this!
My friend if you think that employers only pay money for a visa then you are mistaken
do you know how difficult it is to obtain work visas from the Labour department?
Do you know how much time does it take to process a visa?
DO you know how long does it take to find the right
candidate for your company with the budgets that were assigned?

if you think that its just a mere visa expense then you are mistaken! its way more than that!
I would have only agree with you if the company promises a certain package that you " agreed and signed before you arrive to the country" then it turned out that the package and the working conditions are different upon arrival; then cancelling your contract will be okay!
if the case is that the employee has arrived to the country and started searching for new jobs! then What would I do... I will complete the processing of His visa then cancel it, then ban this employee from coming back to the country for 2 Years...

but that is just me ;)

Regards, Mr. HR Manager ;)