It means that any company can take that code, modify it (as would be required every year per IRS tax changes), and resell it without being required to publish the source code changes.

What many European countries are doing is requiring the government to publish code under a copyleft license. That would allow companies to also benefit from this code to make their own tools (which they could also sell), and it would require them to publish the source code of their improvements.

Basically copyleft legally ensures collaboration. Public domain does not.