Is it acceptable to extend deadlines for coursework delayed due to student activism?
Much would depend on the concrete situation. But, by default, I would think that activism is part of student's life that the student needs to learn to manage themselves; if they do not have the resources or ability to manage their  activism effectively so as not to have them interfere with their studies, then, largely, I would consider this to be their responsibility. One would probably not see any concessions given for extracurricular sports/music activity, and the question is why political activism should be exempt. 

Some may argue that political activism is important as a part of society and therefore of a higher rank than, say, sports or music, but how this is evaluated would probably depend on whose side of the activism one sees oneself. 

That being said, times which are more tolerant of studies as an opportunity for character development would probably give more leeway to activism, while times (such as ours) where studies would be considered more a means to an end (development of employment skills) would probably be less tolerant. I would see the former as an ideal of times past, but the latter as the reality today.