What kind of papers is it acceptable to read during working hours? In academia, one is forced to read papers that come out. Some of the papers I read are directly related to my work and then it is reasonable to read them in my working hours. Others are completely irrelevant and I read them just for the pleasure and to broaden my knowledge in other areas. These, I think, should be read in one's free time.

But then there are papers that are not directly related to what I am doing but may turn useful for my work, e.g., they may use a method that might come handy although they deal with a slightly different field. Should I read such papers at work as well or should I read them in my free time? How do I decide where the border between work-related and leisure-read papers lies?
A perspective from (theoretical) computer science:

One of the things that distinguishes academia from some other lines of work is that there is no well delineated line between "working hours" and non-working hours. Since you can probably be better paid elsewhere, academics tend to enjoy research. You seem to as well, since you read papers during your free time. 

But that is the great thing about academia. Reading papers that you enjoy is work, since as you observe, reading in a field different from your own can nevertheless give you tools useful for your own work. So, I would say that you can read any paper you like during "working hours" and not feel bad about it. 

Incidentally, one can always switch areas. If you feel that you are "forced" to read the papers that come out in your area, but there are other areas that you read for pleasure, perhaps you should work in those areas!

(Disclaimer: needless to say, to have a successful academic career, you can't spend all your time reading -- you have to spend some of your time writing! But there is no need to arbitrarily partition the reading into a work and pleasure pile.)