There's a huge difference between a vent and a crawl spot. A vent is single-file and since humans can't shoot through each other (technically they can with an m249 but it weakens the bullets a lot), it makes sense to limit how many people can be in a vent. A crawl spot is wider and can hold more people safely. The stage in HS and the computer in Miniature Room are both crawl spots but not vents. If they were treated as vents only two people could legally be in them at once because they only have one opening.