Bad design which makes the game worse. Only because it contradicts the established functionality of the game and does so in a way that is totally opaque and feels arbitrary.
It's in the same category as invisible walls on the edges of open-looking game environments, or adventure games with doors that are just part of the background art and cannot be interacted with in any way. Games which resort to these design crutches are objectively inferior to games which do not, and it's always a bit of a shame to see something like this being added into a game which made a name for itself on attention to detail and verisimilitude.
Probably the limit is something they felt they needed in order to narrow the outfitting possibilities they have to deal with, so that balancing the different weapons against the Thargoids would be easier.
There are other ways they could have accomplished this, such as by thoughtful manipulation of power requirements, or by requiring AX tech to have an "activator" device(s) which uses utility slots, or something to that effect. Or even limiting the number of *purchasable* modules per CMDR, on the basis that this is a new technology which has to be rationed until production can be ramped up, etc; but all of these solutions require more work and careful balancing than just erecting an invisible wall.
I don't get the impression that Frontier has a lot of time, attention, resources, or planning to devote to 2.4; so I understand why they need to do it this way; but yeah personally I don't much care for this approach.