Because they knew it was coming, so it made sense to get ahead of the pack with a loss-leader to try and win customers from the networks who were slower off the mark.
Exactly why Three launched inclusive roaming so far ahead of everyone. As the interconnect charges were lowering (and the tiers were known by all), it seemed worth a gamble to get customers.
The One Plan was also a punt, ahead of the industry reducing the termination fees. Now all networks offer unlimited data plans, but unlimited data (and tethering) was crazy cheap - with Three charging just £25 a month at the time, lowered to £15 for a while.
Yes, £15 or £180 per year on a SIM-only plan with unlimited data, including tethering and roaming in the EU and beyond. Total madness (but I was happy to take advantage!) as it did secure loads of customers but was far too cheap to make retaining such customers profitable, and eventually they kicked people off the plan or allowed them to stay on a new plan without tethering for £30 a month.
I knocked it down to £20 which I still pay today, and thanks to the EU and Ofcom implementing net neutrality, I can now tether all my data at home and abroad for no extra money.
Not only do I expect this will end in the EU, I am not convinced Ofcom won't reverse the move and allow UK operators to restrict tethering here in the UK too (it won't be bound by EU law). You can be 100000% sure the operators will lobby Ofcom relentlessly to make this happen.
The FCC caved in on net neutrality Stateside after all. And we love the good old USA!