User blog comment:Morgause5/Speculation and possible spoilers (finale)/@comment-4892425-20121221235721/@comment-5674726-20121222001926

If Arthur had not kept magic outlawed, magical users would not join Morgana's side.

Morgana herself could well have had a change of heart. When Arthur tried to appeal to their former friendship and told her that she couldn't blame him for Uther's sins, she pointed out that it was a little late for that, given that he'd made it perfectly clear how he felt about her and her kind.

Aside from a promise to treat the Druids with respect - a promise that did not keep Merlin from believing that a young "Druid" boy desperately seeking help for his dying sister would have to sneak into Camelot rather than being able to approach Gaius directly, as the people of Camelot do - Arthur has not given those with magic reason to believe that they will be accepted under his reign.

If he wanted to minimise Morgana's support among magic users, legalising magic and granting a general pardon to those with magic for any and all crimes committed before magic was legalised would be an effective strategy. Crimes committed after magic was legalised would be punished as any crimes would - ie. theft using magic is punished in the same way as theft without. This would mean that those with magic would know that, by siding with Morgana (assuming that she didn't cease hostilities in response) they'd be giving up their clean slate and their opportunity to practice magic freely, provided that they didn't harm others.