Few things to consider:

The FW UK champs is not for crowd entertainment, it's unique from the other EOs in that sense.

We want to get as many people as possible involved in the sport, the more that attend the champs the better. Forcing an active weapon rule may alienate new comers, and even vets (like Tiny Toon, Satanix, Bonx etc)

On the other hand, allowing people to build Ubür defensive boxes that'll usurp highly engineered aggressive robots will alienate event vets and they may stop attending.

At the end of the day there's a decent diversity in robot design in the FWs, and if an Ubür box can defeat a monster spinner then maybe it's time to look at the spinner design.
As spinners have become more and more powerful/effective, they've become more popular. Conversely people see this, either build their own, or try to design something that can beat it. It's the natural evolution of things, and it's true in any competition.

I don't think EVERYONE building boxes is an issue we're going to have.