The grammar dictionary どんなときどう使う日本語表現文型辞典 defines がてら in the following way:
- Accomplishing two objectives via one action. Can also mean to produce two results by doing a single action.
- Verbs of movement, such as 歩くor 行く, often follow.
It also gives the following example:
The sentence ② is basically consistent with the example given in this question, and the explanation given in the answer: "the phrase 散歩がてら立ち寄った公園 would make more sense because stopping at a park is done while on the walk."
It is also consistent with the explanation given in this answer: "運転(し)がてら食事する -- eating along the way driving (after having driven half way)"
Now, the problem is that the same dictionary gives also another example:
④ 駅まで３０分ほどかかるが、天気のいい日は 運動がてら歩くこと にしている。
and I see it as contrary to the above.
Walking is neither done in the middle of an exercise; nor is exercising the main action here (as we are talking about the station, then the action of moving towards the station should be, no matter what).
Why isn't the example written in the following way:
駅まで３０分ほどかかるが、天気のいい日は 歩くことがてら運動 にしている。
Would there be a difference in meaning? What would be the meaning then?
Even if, for some reason, it doesn't sound right for a native speaker, please explain what meaning it would convey.
Does it matter that the predicate is making decision (にする) here?