What is the risk of writing in a diary? Why isn't legal?

When Winston begins his diary, he does so secretly, as he is well aware of the consequences he will face if discovered. Keeping a diary is not officially illegal because Oceania has no written law. Ironically, the purposeful absence of laws serves a purpose.... laws would require government to follow them, without laws the government of Oceania can do as it pleases. In Chapter One, we learn that even though keeping the diary is not illegal, Winston is aware that keeping the diary could culminate in his execution if discovered. At the least, he would likely be punished with twenty-five years of hard labor.