It isn't a matter of being forbidden to work in an industry, but rather that some companies may be forbidden to employ you (and even then, the restrictions may only be for certain roles).
Let's ignore the computer/security aspects of this particular situation, and come up with other examples.
If you are convicted of a financial crime, for example, you are not barred from working as an accountant. You can even get a license and work as a CPA.
But some companies may not be able to employ you as an accountant due to your conviction.
And, it is worth noting, you can certainly lose your license if you commit a relevant crime after getting your license (or for failing to report some other felony to the licensing board).
All the above varies considerably in the details between countries, and in the US between states (eg. in some states a conviction for a felony requires a hearing before the certification board before being allowed to take the relevant exam, and the board may not decide in your favor).
Circling back to computers and security, the situation is much more forgiving since there is no relevant licensing barrier; nevertheless, some companies may be restricted from employing people convicted of certain categories of crimes in particular functions: think of having access to a bank's or credit company's customer information and accounts, or a health provider's patient data. But even organizations that are subject to such restrictions have plenty of roles (potentially even security roles) that aren't affected by these restrictions (except by their own internal policies, anyway).