Well, the employer worries that you may get robbed more easily on the bike which is legitimate concern. However, the employer has no right to tell you what to do once you leave work, so they should require you to drop the money off during your working hours, not on your way home, that's your personal time, you may even walk, go shopping, go to the movies and get robbed too. Unless, they pay you for that time so in that case they have the right to tell you how to get to the bank.

So, I would bring up the fact that your commute time is not paid, personal time so if they want you to drop the money off in a safe manner they need to do this during the working hours and provide a car or pay for a taxi cab. Was having a car required for the job? What if you didn't own a car? I don't know your situation, I know getting fired is not fun so you need to decide how much you want to stick to your rights, but as it is right now it's wrong.

Adam