Doesn't sound like they've given you any reason yet that sounds legitimate. A decently maintained bicycle should be more reliable for short distance transportation than most cars and neither would be immune from robbery attempts. So continue to ask for their rationale - but try to do so in a tactful, non-pushy way. I'd also pitch the environmental angle. Their website proclaims a 'Shared Planet' philosophy and says "Environmental Stewardship:
In 1992, Starbucks adopted an environmental mission statement that ensures that environmental stewardship will play a role in all facets of our business. ..." Seems like letting their employees use environmentally friendly means of transport would be one of the facets of their business.