Within some spiritual practices, there are some parts of it that resonate with outsiders, but they also tend to be removed from the belief systems they come from which can be offensive to those practicing them or be seen as bastardizing or desecration of their spiritual practices. I am not a person who belongs to a community whose practices can be appropriated because I only believe in open practices, but even besides the offense and social justice aspects of appropriation, there are even practical and spiritual reasons why you would not want to take from closed practices which I will list, so people have a good idea about what some problems with appropriation actually are.
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Some practices are associated with cultural heritage that you simply cannot understand or appreciate properly if you do not come from that culture. Examples are Korean Shamanism or Native American practices. We may appreciate it as outsiders who see it, but as non-members of the culture we would not have the right context to ever practice it the same way. Deities exist to preserve culture, so appropriating as an outsider also makes no sense from that point of view anyway.
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Some practices are literally dangerous and can potentially lead to interactions with malevolent entities. Some substances like Ayahuasca or Peyote can be physically dangerous if sacraments are not correctly prepared and can literally physically kill you even before we get to the metaphysical side of spiritual appropriation. Others can result in you being cursed or angering deities.
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Secret societies or other groups require initiations and will take offense to the adoption of their practices without the proper procedures. The reasons above can be a part of it, but it is also disrespectful to the group itself and can result in a distortion of its practices. Practices like Kabbalah are used by outsiders and there is a gray area and controversy over what can be considered appropriation, but I would not personally facilitate them within the Cetic faith to avoid any potential problems.
If you ever find that your practices are being considered appropriative by others (This happens a lot with chakras or other concepts from Eastern mysticism) then the best way to deal with it is to figure out what you are trying to accomplish with any philosophy or practices and create/adopt something culturally appropriate but with a similar framework or goals.