A lot of people know about imaginary numbers as a theoretical concept used in complicated math. Many people haven’t heard of illegal numbers. Illegal numbers aren’t a mathematical concept like imaginary numbers, but rather a political and legal concept.
Oftentimes certain binary numbers are banned to be said, shared or used in any way. The combination of those binary numbers that are banned can form a “pure number,” which is just the number it would be if it were a numerical number.
These numbers are often banned or made illegal by governments because they are codes that represent something classified or used by businesses as encryption codes. For example, the code to decrypt a DVD or Blue-ray is an illegal number. A man named George Hotz got into trouble when he published the key (read: numbers) used to unlock the Playstation 3.
Many groups are in opposition to these bans, seeing them as a limitation of their freedom of speech. They point to examples of illegal binary numbers that actually just form bands of color, that could be banned unnecessarily. They display these sort of banners as a sort of flag for their cause. The greatest fear is of short numbers becoming banned and limiting freedom.