Motivation is an important part of understanding politics, and many people in politics do things for reasons that are not always straightforward. Hence motivational questions should be on topic.
There needs to be a distinction between practical motivation (which should be on-topic) and psychological motivation (which should be off-topic). For instance, it would be on topic suggest that a politician supports a policy because a wealthy lobby is donating to their campaign. It would be off topic to suggest that they support it because they are unconsciously rebelling against their father.
Looking at the questions cited as examples:
US bias on Shia terrorists: This is a poor question because the poster does not clearly establish that such bias really exists. However the issue of why the US might be attacking one group while supporting another should be answerable in terms of US geopolitical interests and/or the interests of factions within the US government.
World silence on use of white phosphorus: the current top-voted answer provides a clear explanation of US interests, and other countries could be explained in similar terms.
NATO leaving Syria: again the top answer describes NATO options, their pros and cons, and gives a clear explanation of why the current policy makes sense.
Motivations for Brexit: The question text clearly asks what reasons were put forward by the Leave side prior to the referendum, so it is about political arguments rather than "motivation" in the sense being considered here.