It's more so just a matter of continuing a practice from previously. Part of what happened with the excise framework was that a new framework was announced for the application of excise duties to alcohol products, including wine. Under the old framework, it was an administrative practice that their samples were not subject to duty. This is a matter of just putting it in the legislation, making it explicitly clear legislatively that those samples will not be subject to duty.
On March 1st, 2007. See this statement in context.