What kind of papers is it acceptable to read during working hours?
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.)