[xenocratus]

I have no expertise in the subject, but I assume the weapons stuff can be seen as simple economic relations between the West and Ukraine which is somewhat regulated (like buying food or petrol), whereas shooting down an airplane for them is not - that's a pretty direct war declaration. "Guns don't kill people..." and all that :)

[kavalg]

This. Weapons supply is somewhat gray zone. Russian know about it, don't like it and have openly protested, but it is probably not a good strategy to retaliate for this reason alone. NATO providing intelligence to Ukraine is actually already deeper into the gray zone, but such thing is harder to prove. However a no fly zone means firing weapons at Russian aircraft and is already a direct confrontation that sooner or later will lead to a retaliation. A single incident will probably not lead to escalation (e.g. as the downed Russian plane in Turkey during the Syrian war), however a persistent confrontation that is interfering with their military strategy will definitely escalate things a lot.