I suspect they participate in pick, click, give because it is an additional source of funding.  They are happy to receive 93% of something rather than 0% of nothing via this particular funding channel.  However, they also receive separate funding via other sources where there is not a 7% deduction.

Pick, click, give is a good program for those who like the convenience of giving this  way rather than writing checks.  So is United Way, even with its 11% overhead.  However, since I can do math, and since I know how to write a check and am willing to do so, I choose to by-pass the bundling fund raising mechanisms that charge large fees to handle my money before giving the residual amount to my intended recipients.