I haven’t seen single recommendation by other people which includes any -ve remark. That means every one expects recommendations to be all +ve things. And I don’t think it is reasonable to write only +ve things about a person. It every one is writing only +ve things, what is the need of a new +ve recommendation?
If they have to be of any value, we must be able to write openly about that person. But how many of us can take a open comment about ourself? Can we dare to keep them in our profile?
Also I have seen recommendations which are written by arbitrary people. A write about B, but they never worked together. They may have worked in same company, but never together.
There are exceptions to this rule. I do write recommendations when I feel to write. And for that there has to be a very special characteristic that stands out always. If I say “X” about A then, every one seeing A should be able to see that X easily.