Again, a vampire not targeting friends makes some sense. That said, vampires in this game also happen to enjoy making trophies out of the skulls of the people they kill, so they may not be quite so "friendly neighborhood vampire" if you know what I mean. Furthermore, friends are potentially a liability to someone who has to keep changing jobs every 5 years to keep up appearances, and who would most certainly notice you're not aging. Friends will not be friends anymore when they find out you're a bloodsucking creature of the night, and unless the vampire can indoctrinate the whole relationship list in a blood cult, there's actually even more reason to dispose of "friends" quickly.
As for were-creatures, it's likely that they either don't know when they'll go off (how do you check the moon from 30 z-levels down?) or they're desperately hoping nobody noticed. After all, most players don't know whether a dwarf is infected or not until the first change. Beyond the first change, a scared dwarf may be more likely to just be trying to fit in (as with vampires) than thinking about the good of the fort.
Beyond all that, again, these creatures serve more game purpose than necessarily strict lore purposes. (They're more like placeholder creatures to represent what secrets can do in the future.) Vampires exist to create paranoia, and werecritters are like jump-scares. It's rare to even get a werecreature infection without going out of your way to trigger one, and if you have one, it can be assumed that the player's actions will sufficiently represent whatever isolation procedures are proper.
Likewise, dwarves do lots of stupid, sui- or homicidal things just because of lack of fully-formed AI. Dwarves only recently have started to recognize that fire hurts and is bad and water can stop that. (Forbid-on-death code was added just to stop the lemming rush of dwarves wanting to carry back !!pig tail socks!! that previously could only be stopped by locking down every door between dead dwarves and living ones.) I don't think it was intended behavior in the long run for anything otherwise to be the case.