Each Stack Exchange community has somewhat different norms. As long as a given Stack Exchange community respects the platform (i.e. it doesn't do things like turn Q&A into a debate mechanism), they are free to set their norms as they see fit.
In the case of Cross-Validated, it appears that the preference is that answers not be copy/pasted on the site at all. In practice, this is in keeping with the spirit of Q&A since, if the questions are not duplicated, the answers can almost certainly be customized for each question, avoiding the copy/paste problem altogether.
I would also note that, while the two original meta questions that you cited give a fairly loose interpretation of answer duplication, they are both fairly old, and the current community consensus on this issue is probably closer to that which is advocated by Cross-Validated.
For the current network-wide policy, see:
Is it acceptable to add a duplicate answer to several questions?