Air France and KLM allow you, as Platinum, to pick any seat for free from the moment of booking even on Light fares, because they are recognising you as a top-tier member in their own frequent flyer programme, and not because this is a Skyteam alliance-wide requirement.

Allowing Skyteam Elite Plus members to pick seats for free from the time of booking, or waiving all seating fees, is *NOT* a Skyteam benefit, and you have no valid expectation of any other Skyteam airlines allowing you to do so just because AF and KL do. (It's not uncommon for airlines to give greater "perks" to their own frequent flyers, or from those of very closely associated partners. For instance, Delta and Virgin recognise each other's frequent flyers in ways that are sometimes over and above the basic Skyteam requirements, and which go beyond the recognition and benefits they extend to Flying Blue status pax)