i dont think it is against the law but i knew for example a travel agent i heard of would not be covered by insurance for theft unless she had two people on premises or security reasons.
Of course it's not illegal.
Sure night porters in hotels do it all the time and they're in more danger than a shop worker.
Unless it's in your insurance policy or health & saftey policy, it's not an issue
I don't think it's illegal but an employee working on their own would have some trouble trying to cover everything - what about toilet breaks, lunch breaks etc are they allowed to close the premises during these?