PPC campaign with dynamic bids down only keeps all bids in lockstep.

All of the search terms are in a single category and no other category is possible. If the bid amount is changed for once search term the bid for all other search terms are changed to the same amount.

Seller support tells me that bids for a category staying in lockstep is by design but can’t provide a good reason. The policy seems insane.

I’d appreciate any insight or workaround suggestions.