Since you can only obtain vouchers in limited fashions during the event, there's no harm in keeping the shops open for people to spend them afterwards. Vouchers are soulbound and can't be sold, but this would also keep some venues of getting other items a little more open for people who needed them. It would also let users still be able to use the vouchers in the event of maintenance before the end of an event. Furthermore, you could also introduce new event items to hype users up about them, and get feedback on prices.

While you can technically get additional tickets through different achievements, I personally think that is all the more reason to keep the shops open, or at the very least have the contents and their prices available at all times. Even more so given the fact that you can exchange abundant activity vouchers for other weekly vouchers, and with a cycle lasting 7 weeks (and thus 7 different shops!), that is a long time to remember yourself what the 7 shops' contents and the prices are, and what you have and haven't bought yet, to know which vouchers to exchange your AA vouchers for.

long story short, massive support.