Why do some language-defined attributes have @ and some not?

[Shriramana Sharma via Digitalmars-d-learn]

Hello. See http://dlang.org/attribute. 3 attributes starting at
http://dlang.org/attribute#disable have a @ in front. Apparently safe,
trusted and system also do, though these are documented elsewhere:
http://dlang.org/function.html#function-safety.

Why are some language-defined attributes starting with @ and others
not? Wouldn't it be consistent and less-confusing to, say, only use @
for user-defined attributes and not these language-defined ones?

-- 
Shriramana Sharma ஶ்ரீரமணஶர்மா श्रीरमणशर्मा