I am baffled by this question. Elsevier boycott aside, is one supposed to review a paper every time he/she is asked? And somebody will be upset if he/she declines the request to review? Or is this only maths thing?
Reviewing paper is unpaid, while it takes a lot of time and effort (at least for me). As such, I expect someone in editorial board to be thankful if one accepts to review a paper, and move on if his/her request for reviewing is declined.
To be angry with a (junior) researcher for not reviewing a paper is unacceptable behaviour for me. We are not slave.
Note: Until now, I am happy every time I am asked to review a paper.