[randomdata]

Unless this attracts the desperate who wouldn't apply without these filters, the compensation necessary to secure the winner will be unaffected or might even go up as the smaller pool of applicants will have Canonical over a barrel.

More likely they do this because they have so many applications coming in that having to pay more to a lesser worker is worth more than having to hire many more HR staff to process all the applications. The hope is that it will scare off most people, and if that fails a machine can filter on these values. It could be any arbitrary value – race, religion, random number – but courts have effectively ruled that any filtering mechanism that isn't directly related to the job or school could face severe legal consequences, so this keeps the lawyers happy.