The point was why do that on a forum where the majority think he is guilty and are no way going to give him the time of day let alone any money?
Just because those who actually post on red now at this point largely think he is guilty doesn't mean that all do and doesn't mean there isn't traffic from people who think otherwise. Mos who think he is guilty probably won't post but they still can read posts. A small minority of people who view posts actually take time to post on a site.
Maybe she thought Mike would delete it here. Maybe she thought it would be drowned out by long attacks from people here so that the initial point would be lost.
The speculation of why could be endless and is not very important. When people feel swindled and conned they warn others not to fall into the same trap. She obviously invested a lot in their relationship and thus it disturbed her to find out she meant nothing to him but a source of money.
Consider the following different scenarios;
1) you contribute money to a charity that wound up being fraudulent
2) you contribute to a charity where you sponsor a child and exchange letters and photos with your "adopted" child and are develop an emotional attachment with the child writing he/she loves you and about personal things going on in his/her life. You eventually find out it was a fraudulent charity and that there was no such child the whole things was made up and the letters were written by con artists.
While in either situation you would be annoyed at being taken in, the latter is much more disturbing because of the emotional investment. In the latter situation people often will say it must be a mistake and need to be beaten over the head to realize it.
Daisy felt more like the latter situation and wanted to warn others not to become emotionally invested and to be careful about financially investing.
You can see why she could expect being attacked here and possibly having the post deleted.