- Different sites might have different standards. Our rules for referencing other materials which requires the use of
> to indicate citation can be found in help center here. The guidelines show example of referencing material and our guidelines explicitly state:
Do not copy the complete text of external sources;
Hence, not only you should not just post a print screen of the text but you should also not provide an answer that is just long quote from the text.
Pictures are appropriate only when you are showing graphs/figures/tables.
Also we are actually relatively laid back about these rules. Short snippets for equations are often tolerated even if our policies in help center state you should be using MathJax formatting - which is important as it makes the equations searchable. In addition we also usually tolerate when short snippets of text appear around the graph/figure/table (e.g. their captions, short text in the box where they appear) or even around the equations if they support proper answer, although such practice is still discouraged, and even if it would not lead to moderator intervention it might lead to downvotes/negative reception from community.
However, just taking print screen of a whole page and posting that as an answer is simply too much and it crosses the line.
- Actually those examples you show are not good examples of Mathematics.SE question/answers. For example, under the answer 2 you link you will find the following comment:
Please do not use pictures for critical portions of your post. Pictures may not be legible, cannot be searched and are not viewable to some, such as those who use screen readers. Scanned pages from books are discouraged on SE network. Questions should contain sufficient context so that it is answerable with the text alone.
You can also see that all the questions/answers you linked have little upvotes indicating that most users do not consider those post of high quality and it is very well possible they just managed to 'flew under the radar' of the mathematics.se moderators.