Why should they? There have been cases where staff have been stalked by passengers that i'm aware of (this is due to a surname being visible on an ASIC (which BTW isn't there for passengers writing down names)).
For at least five reasons:
1. Basic (Christian or just commonsense) courtesy. What's good for the goose...
2. Staff (including you) have our full details (including in some cases access to 'sensitive' items such as dates of birth, occupations and email addresses), so why shouldn't we be at least have your first name and surname? If staff are going to behave in a completely unacceptable manner to harmless passengers on any conveyance, why shouldn't they be as accountable as badly behaving passengers are?
3. On some airlines on which I've been, the captain, first officer or other flight crew member making a PA welcome announcement prefaces it with 'good morning, this is First Officer Boris Smith from the flight deck...' or similar, so if they have no objection to giving out their surnames, why do cabin crew or (to use the USA term) 'gate agents' on QF seemingly have such reluctance, or treat it as a 'security issue' when someone asks for their surname?
4. I and many others have always given out our full names to numerous individuals we've dealt with, and in in completely different sectors to aviation, it's led to the very occasional death threat, or more commonly, complaints (some of which are justified, and others frivolous or exaggerated, but that's life.) This hasn't meant refusing to disclose full names, because anyone who really wants to can - eventually - find this out any way, although perhaps not as quickly as the hip hopper did.
5. A failure to disclose these very basic details gives one the impression it's to make complaints harder to pursue, or to allow staff to get away with unacceptable behaviour more easily.
That said, I'm mystified how the hip hopper found out the lady FA's surname so expeditiously.