No, this is not legal. No cemetery or funeral home can demand that you purchase a memorial from them. Many of them will try to talk you into purchasing a memorial from them, or their own vendor, but by law, they can not demand it. You are free to choose your own monument builder.
Many of them will claim that that the monument is not guaranteed if you do not purchase it from them. This is not true. The guarantee is provided by the monument builder, not the cemetery.
If you have any problems with a cemetery or funeral home concerning your monument builder, please let us know. We will contact them on your behalf and resolve the issue.