Absolutely sure that Bournemouth International Airport Ltd owns the business park, on a very long lease from the Council. I believe that a number of the newer buildings were built by tenants who leased the land and paid for their own building. The older (WWII) building stock is owned by the airport.
Why invest in the business park? Because it's an enormous investment opportunity in an area with high property cost and demand. It depends on your definition of 'acceptable level of return'. My point is that in the last 5 years MAG has invested little in either the business park or the airport and that suggests a strategy of maximising short-term returns at the expense of investing for the future.
Whilst pax numbers at MAN might be falling, I'll bet that 1 pax at MAN is worth more than 1 at BOH, both in landing fees and retail. And so spending a pound at MAN to retain a passenger is better than spending a pound at BOH to gain a new one.