Not realistic, so not acceptable
Why wouldn't the goblins sneak their sapper into YOUR fortress if it's wealthy enough?
It's better than try to conceal a siege convoy through a dwarven civ
My excuse for goblins being able to dig only in evil zones is that perhaps they have more resources in such areas.
It may not be realistic, but not all gameplay elements are required to be realistic. This difference in evil and non-evil areas should satisfy the types of players who choose such areas in which to play, or so I would hope.
Evil areas, with their skeletal elks and zombie eagles, generally seem to be the domain of challenge seekers. My guess is that they would be less likely to object to sieging civlisations being able to tunnel.
Players who seek to build in peace generally will choose non-evil places to play and seem more likely to prefer not to have their fortresses disturbed in such a permanent manner.
Therefore, I believe that making digging siegers part of evil biomes only would cater to both sides of the digging siegers issue.
It would be nice, too, if one were able to refill the soil areas, but I guess that feature will come around later.
Perhaps we can restrict tunneling to "evil" areas, since people who build in evil areas tend to be the ones who are seeking a challenge. This way, people who like peace can continue to build in neutral and joyous areas without invaders disrupting the integrity of the land and players' fortresses.
Would this be an acceptable compromise?
Well, I like 'build well within the confines of your civilization and you are safe, build on the borders and it's more risky' style of play. 'Evil' areas seems to be gamey but placement would be a good option.
This idea is a better one. Playing too close to a goblin civilisation would, indeed, be asking for advanced sieges. A proposal that is both realistic and gameplay enhancing.