It should go without saying, but zoo animals are kept inside enclosures for a reason, and that reason is to keep everyone (including those animals) safe from harm.
You aren't only putting yourself in danger when you decide to forgo the warning signs and hop the fence at your local zoo. If anything happens to you in there, it's very likely that animal will be put down simply for reacting to your intrusion. Which really isn't fair at all.
Although zoo rules are super clear, apparently not everyone feels like they need to abide by them.