Have you read all of the documentation for the sp_dbextend stored proc?
The 'default' entries are created by the installdbextend script and act as default configurations to be applied across all databases ... with database-specific configurations having higher priority. If you don't want the default settings to apply to all databases then disable them (see the documentation for sp_dbextend).
As for the question 'what would happen ...':
1 - What does the documentation for sp_dbextend tell you will happen?
2 - I've already suggested you look at the sp_dbextend 'execute' option; did you read up on that option? Did you try running that option?
3 - Instead of asking what will happen, why don't you try it and see first hand what happens? ("When in doubt, try it out!")