India has a chronic problem of tax evasion, in the grand schemes of things only a handful of their citizens can claim a defense/offense of "it's ability to tax foreign income of resident Indians", since only a handful do not have the ability to evade taxes or have never deliberately evaded taxes.
A quick search for "tax evasion in India" can educate on this topic, including an article[0] that states "tax evasion is India's national sport."
My Indian friends tell me, that they don't feel bad evading taxes a little since the politicians, bureaucrats etc would squander it through bribes and scandals. To which I say, perhaps the main reason for rampant scams and bribery is that taxes are evaded and these public employees are paid poorly.
[0] http://www.businessweek.com/magazine/in-india-tax-evasion-is...
edit:
On re-reading my comment I realized that I was unclear why in it's current scope RBI's main concern is not the scope of tax evasion. Currently India looses over $300B to tax evasion each year, even if all of bitcoin's market cap was taxable by India this would be a very small fraction of their tax losses.
I believe the real reason lies in addressing customer grievances. India is plagued by numerous scandals and scams in the private sector and the only resort for most people is going to the consumer courts if they can afford it. Their already few options would be obliterated if they choose to deal with bitcoin.
While this may be a concern in the developed countries, its not nearly as bad as the situation in India and similar economies.