I spoke with our network team.

More or less, this is our solution with the resources (time, money, workhours) that we have. If we assume that India has the same percentage of risk as the UK or US, they still pose a greater risk in absolute terms.

If 1% of all traffic from those countries is malicious, that's 700k from the UK, 3.5M from the US, 14M from India. That's quadruple the next highest country (since China blocks us).

I am not here to defend any corporate decision in that regards, I'm just conveying the reality and mindset that people hold at IB who are more knowledgeable than me in this particular domain.