pdtraveller is absolutely correct – it is all about money and responsibility.
BA, in this instance, would technically be responsible for the baggage. On one ticket, they cannot decline to ck the bags, and as part of an alliance they tend to take them as well (even if on two tkts). But had Ian turned up in HKG with no bags, it would be BA holding the responsibility and cost for finding the luggage.
As Ian discovered, some carriers are willing to take the risk, like TG. Others, like BA and others, will not.