Unlike every other champion interaction with terrain, it's only out-of-combat. It doesn't make her pathfinding any more predictable or exploitable compared to other champions. It only serves as an unnecessary complication to triggering it.
Obligatory tweaking is obligatory. Unrelated balance arguments and complaints about Taliyah (or otherwise) are unrelated.
Taliyah passive might be too strong if it triggered away from walls as it is 20-40% ms based on level.
Having her passive work away from walls would detract from it's purpose as a roaming passive because instead of Taliyah having to be near walls to trigger it(conditional), Taliyah could use it in lane and get a useful speed buff by just not attacking minions for 5 seconds to use her w on her opponent.
Sure, Riot could lower it's speed to balance it, but then, why not just give Taliyah a new passive because having a weak Ms passive is not as useful as a strong wall Ms passive, because Taliyah's theme is roaming.
Tldr: moving it away from walls makes it less of a roaming passive and makes it into a generic movement speed passive (like ghost blade).
