I do this too, and lots of companies also don't allow you to use THEIR company name in the email address that you give them.

Sometimes their front-end is not aware of this restriction and will let you register, but then you'll have unresolvable issues. I've spent some time on tech support phone calls with companies that have this issue.

Occasionally a company will implement blocking their own name in a user-provided email after the user is already registered. I've had this happen a few times too. Suddenly the account will disappear without explanation.