Hmmm, a miscellany of thoughts:
In the UKAs I understand the law, this would probably be legal - in general, you can build what you like in your front garden, as long as it's temporary. If it's permanent (e.g. an extension to your house), you need planning permission from your local council. Of course, if the construction features something obscene, or an incitement to violence, or libellous, then legal action is likely to follow quite quickly. Other constraints might be a local by-law (fairly unusual, I think) or a contract between a tenant and a landlord.
In the USI'm not so familiar with the detailed legal position, but, as CG implied, the
First Amendment provides a lot of protection in this sort of situation, in terms of freedom of speech; there might be some argument based on the freedom of religion clauses as well. On the other hand, I suspect that local authorities (as Sycamore Township appears to be) have more freedom to introduce local laws than they would in the UK, hence the mention of permits. It would be very interesting to know what would have happened if the permit conditions had been strictly adhered to - this would have made any legal case much clearer.