Why would God make plants that are poisonous if He said to Adam and Eve they could eat all the plants?

Answer:

"And God said, "See, I have given you every herb that yields seed which is on the face of all the earth, and every tree whose fruit yields seed; to you it shall be for food" (Genesis 1:29).

The command is not quite as broad as you indicated. Their diet was seed-bearing plants and fruit trees. Poisonous plants did not come until later in response to man's sin. "Then to Adam He said, "Because you have heeded the voice of your wife, and have eaten from the tree of which I commanded you, saying, 'You shall not eat of it': "Cursed is the ground for your sake; in toil you shall eat of it all the days of your life. Both thorns and thistles it shall bring forth for you, and you shall eat the herb of the field" (Genesis 3:17-18). Prior to man's fall, food was readily available. It didn't require cultivating. After the fall, man's food had to come from the field. In other words, he had to work at cultivating crops to have sufficient to eat. But along with this, there would now be plants that would hinder production or not be good for food. Thorns and thistles are given as examples of these, but it would also include poisonous plants.