It's the same case with many council airports now listing in ERSA that the grass portion of the runway (that's required to be suitable for ground run of aircraft under the MOS for Part 139) is not to be used for aircraft operations.
I guess that's because, strangely enough, councils would prefer that pilots use the (from the MOS)
Runway
A defined rectangular area on a land aerodrome prepared for the landing and take-off of aircraft.
for landing and take off, because it is built for it, rather than the
Runway strip
A defined area including the runway and stopway, if provided, intended:
1. to reduce the risk of damage to aircraft running off a runway; and
2. to protect aircraft flying over it during take-off or landing operations.
which is not. Yes, the Runway Strip is suitable for the ground run of an aircraft to the extent that it reduces damage, not prevents it, however the surface standards are less stringent than a Runway, as you are only meant to be on it if you have run off the Runway (as per the definition), which I guess required explaining the runway excursion you had just experienced to the ARO in the first place. Electing to later take off in weather conditions that precluded you from using the actual runway? Safer?
Also, how did you know that the ground under the grass was safe to land on? There are no bearing strengths dictated for Runway Strips. What if you had left deep ruts in the strip, which the Aerodrome operator then has to repair, or got bogged within the Runway strip and closed the Aerodrome until you could be extricated? What if you had hit a soft spot on takeoff that retarded your acceleration and compromised your takeoff? What if you hit the PAPI boxes or TVASIS boxes in the grass that you missed seeing over the nose of your tail dragger?