I have this question for long too. I think its not just a gimmick though. Because gimmicks don't work equally on consumers of all cultures equally. Fruits and vegetables rarely end with highest possible number, I.e..; 9.
But electronic goods and other scannable products do. I think it is for the convenience of the machine that scans. 10 is the smallest possible number or I should say tens are the smallest possible numbers and 9s are the largest possible numbers.
For example in 1-100, 10 is the smallest possible two digit number and 99 is the highest. Maybe these machines are set to scan highest possible digits easily. It's just a speculation I have.
One more idea I have is that it is some kind of deduction in form of tax or insurance. This one rupee on every product is essentially a profit to the seller because we don't get our change in return. This is not shown anywhere. Since a lot of products are brought, there is a lot of one rupee as money circulating.
If that is the case, then it's a pretty nifty strategy.
The stronger a light shines the darker are the shadows around it.