I found this reference interesting because I had not thought about the Romans engaging in trade protectionism before. I did a little more research and found that the prohibition of the cultivation of grape vines beyond the Alps was actually mandated by the Roman Senate as early as 154 BCE.
"It appears from the newly discovered treatise of Cicero de Republica, that there was a law of the republic prohibiting the culture of the vine and olive beyond the Alps, in order to keep up the value of those in Italy. Nos justissimi homines, qui transalpinas gentes oleam et vitem serere non sinimus, quo pluris sint nostra oliveta nostraeque vineae." - Footnote from Gibbon's Decline and Fall of the Roman Empire.
Mulsum was wine sweetened with honey, mixed in just before drinking (and therefore not like mead) and served as an aperitif at the beginning of the meal. (Conditum had herbs and spices such as pepper added as well.) Often freely dispensed to the plebs at public events to solicit their political support, the demand for mulsum became so great that it was more profitable to sell wine at home than to export it and, by the first century AD, wine had to be imported from Iberia and Gaul. - Wine and RomeThe problem of a wine shortage was further exacerbated by Domitian's vine edit in 90-91 CE.
"[Domitian]...thought that the corn-fields were being neglected as a result of too much concentration on vineyards, and issued an edit that no more vines were to be planted in Italy and that vineyards in the provinces were to be cut down, leaving no more than half standing..." - Suetonius, Life of Domitian, 7.2Suetonius goes on to point out that Domitian did not initially go through with this measure when a poem appeared depicting him as a vine-eating goat. But he was later praised by others like Silvae (4.3.11-12) who hailed the legislation saying "to chaste Ceres, restores acres long denied her and a sober countryside", obviously considering the legislation more moral than economic, as did Suetonius and Statius.
"Undoubtedly, he wanted to encourage cereal-production both in Italy and in the provinces for shortages were not unknown. The vine edit (of c. 90-1) had been issued just before the severe famine in Pisidian Antioch (92-3) but there was more to it than this; the cities of Asia had been growing and their populations expanding for many years, but the emperors were reluctant to allow the local elite to assume complete control over the production of corn [grain], for fear it would result in a concomitant loss of imperial authority. In fact, what it did produce was famine. Domitian tried to deal with the problem: hence, perhaps, his popularity amongst the provincials." - Brian W. Jones, The Emperor DomitianFurthermore, the cultivation of grapes in the provinces was apparently restricted to Roman citizens until Caracalla conferred citizenship on all free inhabitants of the empire in 212 CE. Then, the last restriction on viticulture, Domitian's edict, was finally overturned in 280 CE.
In researching this aspect of Roman law, I found one other little interesting bit of information. I knew that Falernian wine (made from grapes grown on the slopes of Mount Falernus on the border between Latium and Campania) was generally considered the best quality. A wine price list still visible on the wall of a Pompeii tavern declares, " "For one as (a unit of money) you can drink wine; for two, you can drink the best; for four, you can drink Falernian."
"Next in rank were the wines of the Alban Hills southeast of Rome, and Surrentine and Massic (among others) from the Campania. Finally, there was Mamertine from Messina, first brought into favor by Julius Caesar, who had it served at public banquets." - Wine and RomeApparently Julius Caesar spent lavish sums of money on public banquets but even he did not favor the mob with the best wines Italy had to offer!