They .Ake it hard because they want to ensure your saw is properly taken care of...quality parts not cheap knock off. They want to ensure you as a customer have a good interaction with a parts source, ie dealer, and ensure their dealers are supported which allowing online parts sales undermines.
No aftermarket company like stens is an OEM supplier to Stihl, or save 1 specific case I know of, anyone else. Those companies sometimes source OEM parts...carbs from Zama and Walbro for example. Otherwise they have simply stocked the latest China made copy of a part in the interest of providing a smaller service shop who is not a dealer for a specific brand access to parts compatible to fix customer equipment.
Fwiw, there are no piston cylinders available for an 016 now, the saw is obsolete. Piston may be available if they still use a compatible size in another engine, rings the same, but gaskets and cylinder are nla. Almost all 0series saws are obsolete and cylinders are nla.
Stihl aggressively pursues online parts sellers as its a direct violation of dealer agreements.
Source: Stihl trained tech in a dealership.