The idea is actually quite simple. Say I issue this ultimatum: vote for who I tell you to or face a permanent ban from this site (just as a quick example). To prove that you have complied, you might provide a photo of your ballot at the polling place. If such a practice is illegal, I cannot make this threat. A much better example than a schmuck volunteer moderator of a gaming site making such a threat would be someone with money attempting to buy votes or an employer threatening termination if an employee doesn't comply with the company's preferred candidate.
Mind you, I get that it's kind of pointless. In a world of PhotoShop and sample ballots it'd be pretty easy to fake so why would someone even try to demand a photo as proof? But I understand the idea of discouraging the practice altogether.