Why would forfeits in football be scored as 3-0 to the non-forfeiting team?
Is there any reason for this score?
If the final standings depend on goal difference, goals scored, goals conceded, or a score between two specific teams, then a score needs to be recorded.
Clearly it would be a benefit to the team forfeiting to be granted any goals, as this helps their goals-for. On the other hand, if a team would only get goals by cheating, they shouldn't be given any. Thus, the forfeiting team gets a score of zero.
The team that wins shouldn't also be given zero, since a bad team could just enforce a nil-all draw (in terms of goals scored/conceded) with anybody better, which does more damage to ranking than anything else might. If they're one of those good teams, a single goal isn't fair either, but neither is say, twenty (since it causes a similar problem).
So it has to be something sensible, something realistic. Now three goals is pretty decent, and affects the forfeiting team's goals-against more than just playing the match (ideally, anyway), acting as a deterrent.
You'll find some competitions will give five-nil scores instead of three-nil, or will use a formula to estimate a "more accurate and fair" winning score. Three is an overall guess at the average expected score if the forfeited match were actually played, as it
- removes extra thought
- removes additional oversight requirements
- is easy and simple to remember and apply as a rule.