You're welcome!
Why would O2 care about tethering?
Because you are far more likely to use your full 25GB allowance via other devices than you would be if you had to use it all on your phone. Offering you 25GB/month is one thing, delivering it to tens of thousands of users is quite another. They hope that most users won't use anything like that much but will keep paying the high monthly fees.
For reference: I have had a "no tethering" contract in the past (Vodafone), but my present one permits it (also Vodafone).