From my understanding this isn't a last minute new rule that the FIA have suddenly put in, the FIA just clarified a regulation that the teams either weren't aware of or didn't understand. The testing rules have changed since last season since the FIA are allowing an in-season test at Mugello May 1st - 3rd.
Anyway, we criticize the likes of HRT & Marussia for not being able to build a competent car in time for the pre arranged tests so why should teams like Lotus get to do private tests if they've put themselves in a similar position? Red Bull claimed to want to change their testing dates for 'logistical reasons' despite knowing about these test dates for a long time. I just think that after Mercedes got their private test day, Ferrari & Red Bull want the same but either everyone should get to privately test or noone.