I don't have any experience with wheelchairs, but I would imagine it would take years to wear out a tire, vs a bike tire that for most long distance cyclists will unlikely last 6 months. Rubber deteriorates with age (tho' I've never noticed it with tubes). If the consumer is changing his own tires, sure... keep the tube. If they're throwing down to get it done in a shop, they don't want to have issues with the tube in 6 months.