It's not that they don't allow them, it's that they have to add the mods to their server which takes time, can have a lot of bugs and they won't be able to keep up to date with the Minecraft versions. Mods take time to update to newer version, so servers would have to wait for it to be updated before they can then update which defeats the point of a public server.
These trap mods are designed to work with other mods, I'm running a modpack which has very dangerous mobs outside, without traps around my base it would be destroyed and riddled with insanely difficult mobs. Not many mods are designed to work on just a standard vanilla minecraft world, to experience them properly you need other mods with it.