But as a PP said, the staff have a duty of care to everyone there and although we and they know that, if your dad was being threatening, it's the dementia rather than 'him' as he was, but in the moment that's not actually useful to them when they need to think of everyone's safety.
They have to manage the behaviour regardless of the intent behind it, and by giving him a choice that he either stopped behaviour that was unsafe/unfair on other residents OR went to his room, they actually handled it well (IMO). They gave him agency, made clear the behaviour wasn't appropriate as it was, and ensured calm/safety for everyone involved. A lot of shitty care settings wouldn't have taken the same approach.
It's tough - I helped care for two grandparents with dementia and then lived with my best friend's family, including her grandma with dementia, for years, so I know how much of an emotional minefield it is (and how terrifying it can be when someone is no longer in your care but that of strangers).
In an ideal world, good dementia care would have plenty of well-paid staff who were highly trained in caring for people with dementia to allow for a high staff:resident ratio, it would give residents agency rather than treating them like children; and it would have the right balance of familiarity/routine and engaging activities. The specialist dementia home my grandma was in before she died was excellent - it wasn't just rooms full of people sitting pointed at a TV. Staff engaged residents (where possible) in cooking, baking, tidying up, gardening making crafts, sensory/soundbath/meditation-type stuff, they had masseuses and dementia-trained hairdressers visit, they played music from various eras on different days and did little 'dance' evenings where they did a few steps of proper ballroom moves and got people up enjoying themselves, showed old films to spark memories in people, had an open door policy for visitors, and had a good number of staff who had endless patience and talked to residents like people rather than toddlers. I loved visiting her there - it cost an absolute fortune and was worth every penny.
My grandma noticeably improved in mood and distress levels there compared to the previous two places which were staffed by overwhelmed teenagers and where the only activity was 'TV'.