Indeed, service=alley in Vietnam is normally for building front access. There’s a simple explanation for why they’re tagged as alleys: Alley 1716/46/24 is thought of as an alley, and an alley is a kind of road, not a kind of path or trail. (You can even write “Alley” instead of a slash when addressing an envelope in English.) Typically, a narrow alley connects to another narrow alley to another narrow alley to a grand motorcycle boulevard. Sometimes mappers arbitrarily promote a more accessible alley to highway=residential, but this isn’t based on any particular aversion to highway=service.
If this differs from analogous things in Thailand, perhaps there’s a linguistic reason for the discrepancy?