The problem with making those under level 5 join a level 1-4 server forcefully is that you never quite know when you’ll get an influx of new players. It may work for current player numbers, but if a large number of players join Dirty Bomb at once, it risks not having enough server space for them thereby losing players. This will then leave SD with a choice - either increase the number of max level 5 servers or lose players; the former option thereby either causes a possibly unrequired cost expenditure or for current open servers to be switched to max level 5s, which is detrimental to those who might be above that level.
Levels 1-5 should preferentially join one of the max level 5 servers however, if they don’t already.
I totally agree that it is annoying though.