I don't know the details, but I can imagine there are lots of cases where contamination is "fine" (read - below a somewhat arbitrary threshold) when the ground is left alone or only minimally disturbed but where its not "fine" to have heavy equipment running over it, digging it up, etc.
Edit: This is one of those cases where I feel like just because we don't know the reason, doesn't mean there isn't a good one. Sure, sometimes its something 'silly', but more often than not the people that know what they're doing actually know what they're doing. At least in terms of the people that say the contamination would need to be cleaned up. The decision of where to put the TPSS and money to be spent on cleaning up the contamination may be a totally separate matter.
Edit: This is one of those cases where I feel like just because we don't know the reason, doesn't mean there isn't a good one. Sure, sometimes its something 'silly', but more often than not the people that know what they're doing actually know what they're doing. At least in terms of the people that say the contamination would need to be cleaned up. The decision of where to put the TPSS and money to be spent on cleaning up the contamination may be a totally separate matter.