Meditation is about tuning in. Dissociation is about tuning out.
Appealing way to put it, but I'd say their relationship is more sophisticated than being opposite.
Meditation is about concentrating, paying attention to everything happening in your mind and in front of you but not engaging. So I wouldn't say it's as simple as an activity or passivity.
Dissociation ultimately leads to anaesthesia or amnesia etc, so indeed you tune out. But before you are completely tuned out, it's possible that thoughts are quelled allowing you to pay very pure attention to what is happening. However I found the Zen headspace from dissociatives (e.g. 3-MeO-PCP) to be fake since your ability to concentrate better only comes from being partially impaired cognitively, artificially.
That said I do think there is a cunning likeness between the states of consciousness where this seemingly paradoxical activity vs passivity (i.e. not running in circles after your own endless thought stream but just being there, sincerely and compassionately), even though they ultimately lead to very different places: clearheadedness for meditation vs anaesthesia / amnesia etc for dissociation.
I've found intense / prolonged meditation to be not unlike a light dose of LSD, with the effects happening naturally and at the pace of your own mind instead of catalyzed by the LSD. Psychedelics like LSD and also meditation do have a dissociative tendency and I believe it's part of the meditative process, just like perceptual distortion can be part of the meditative process. They are just side-effects along the way though, and have nothing to do with where you are led beyond them.
The reason probably has to do with how the mind reacts to phenomena like sensory deprivation, because (while also walking meditation or mantras and many other forms exist) the peaceful / monotonous or repetitive nature of the non-activity during meditative practice has effects that are pretty much brain-washing without writing on the tabula rasa. It's a great way to prime the mind, and brain-washing has such nasty connotations because of how it can be abused... but clearing up your mind tends to have great potential in general.