Basically, Kelp can't be placed on magma blocks but it can be placed on Soul Sand. It seems intentional that Kelp can't be placed on Magma since it would conflict with bubble columns working properly. Soul sand also has the problem that it conflicts with bubble columns meaning that kelp shouldn't be placeable on it.

Alternatively, maybe it is fine that kelp conflicts with bubble columns. If it's the case then kelp should be placeable on magma.