Saying it again, the 5,000 DOT is not targeted at Ambassadors. Every track in OpenGov has a decision deposit. The design of OpenGov is not “unintentional” in this regard. Repeating again, Polkadot is a crypto-economic system.
The Fellowship was seeded using an on-chain migration to initiate the accounts, as the Fellowship manifesto stated in its seeding process. The referendum that approved the Ambassador Program specifically stated that there should not be an off-chain seeding process, but rather each HA would have to apply individually.
That said, some ranks in the Fellowship (e.g. VII and above) are only attainable via public referendum, see for example the ongoing one to promote Gav to rank VII. Someone, I don’t know who, placed the 5k decision deposit.
For HAs, if they have the skills necessary to be effective as HAs, I’m sure they can convince someone to place the 5k deposit to support a decision on their application.