Mar 1, 2017
Why does doubling season not put additional counters on a planeswalker when activating its loyalty abilities?
Because those counters are being placed by a cost, not by an effect.
In Magic, an effect is defined as anything that happens as the result of a spell or ability resolving. Some things that aren’t effects include special actions, like playing a land, and paying costs like the +1 of Garruk Wildspeaker’s first ability.
The cost to activate a loyalty ability of a planeswalker is to add or remove an amount of loyalty counters as indicated by the number before the ability. Since these counters aren’t added or removed by an effect, Doubling Season’s replacement effect doesn’t care about them.