This is a constraint of SQLITE, not phpLiteAdmin.
"If a NOT NULL constraint is specified, then the column must have a default value other than NULL." 
And this makes very much sense. You are adding a column to a table. In this table, there might already be some rows. When the column is added, what should the value be in these rows? It must not be NULL, because you specified it NOT NULL, but you also did not specify a default value, so what value should it be? SQLite does not know and therefore rejects the query.
Of course we could say in the special case that the table is empty, we provide a workaround.
I have to think about this.