where there's a suspicion for whatever reason that the employer is employing illegal immigrants AFAIK, though there may also be an element of random checks on employers.
at the moment there are essentially 3 immigration policies on the table, lib dem, labour and conservative. They are basically the options available to the country after this election, so by attacking the lib dem policy you're making it more likely we'll end up with either the labour or tory policy.
remember the other main element of lib dem immigration policy is an amnesty for all current illegal immigrants in the country, so we'd basically be starting with a clean slate on immigration and trying to set up a system that works properly from this point forward rather than maintaining the current fiction that condemns hundreds of thousands to working in the black economy.