I think it's interesting to notice that his amicus is responding to a specific request from the court about a specific section of the ADA Standards.  Notice that it does not address any of the other requirements in the ADA regulations such as:

1) 36.211 - The obligation to maintain accessible features in an operable working condition.

2) 36.302(a) - The obligation to modify policies, practices, and procedures, when necessary to provide access to goods and services unless the public accommodation can demonstrate that making the modifications would fundamentally alter the nature of the goods, services, facilities, privileges, advantages, or accommodations.  Providing sufficient space to allow people with disabilities to fully and equally use the counter would certainly not fundamentally alter the nature of any of these.

3) 36.304 - The obligation to remove barriers to people with disabilities when it is readily achievable to do so.  Note that this requirement specifically applies to movable elements and is a broad obligation that would clearly apply to display items on a sales counter at the point that any of them become barriers.