I agree with Aleflippy. In the same way only Christmas Zeds appear on Santa's Workshop, they should do that for all themed maps that come out during events but keep the Zeds on those maps only. For example:
Summer Sideshow Zeds: Tragic Kingdom
Halloween Zeds: Nightmare, Monster Masquerade
Christmas Zeds: Krampus Layer, Santa's Workshop
Of course, I don't think they should always put the themed Zeds on each map with the season each map comes out with. For example, Christmas Zeds should not permanently appear on Elysium.