This is pure speculation - but it is probably important to our social status that we are recognized for our contributions. If others can shamelessly take credit for all our brilliant ideas, they will increase in social status, while we seem not to contribute anything. Thus it makes sense that we would have a deeply seated, hardwired resentment for other people taking credit for our hard work.
Even if somebody copies you and then attributes the work to you, some of the credit for the idea still goes to that person rather than to you. Attributions are often overlooked by 'the public' and the social status often still goes to the person they got it from.
The power of this kick in the gut may lessen the more comfortable you are with your position, and how well esteemed you feel by the people around you. You may be less opposed to having your work copied if you feel, on a deep, emotional level, that you are still in general highly respected for your contributions even if you 'lose' a few to other people copying them and taking credit.
Which reminds me of this comic by Jessica Hagy: http://thisisindexed.com/wp-content/uploads/2010/12/card2752...