Actually, the corporate editions are licensed for one year. That doesn't mean it stops working after a year, but you as the network admin should know you need to update the licenses.
Some versions (the antispam/email antivirus) will stop updating if the license file isn't up to date. Other versions (ie corporate desktop versions) will continue to update, although you may be in violation of your license agreement.
Just because it updates doesn't mean you're not in violation of the agreement. The licenses are extremely cheap on TechSoup and there really shouldn't be a reason to not stay compliant.