Fine hit the nail on the head
in the "How would you respond post". This point is basically the crux of most issues the step parents on this board have.
**"How can any mother be upset when her child has good time? with anyone, dad, grandma, friends? even if she hates dad, doesn't she love her son?"**
The simple answer is, because they are selfish. Because they care more about their feelings than those of their child. Because it makes them feel bad, so they knowingly make an effort to make their child feel bad as well - punish their child, if you will, for hurting them. We all know that is not what the child intended to do, but unfortunately for the extremely self centered this is not so obvious.
Truly, many issues stem from that way of thinking and parenting.