Main reason for denial of a Visa by the US is that the applicant couldn't provide enough evidence that e or she will return to his/her home country and does not want to immigrate to the US.

When you want to get a temporary non-immigration Visa for the US you have to proof that you will return to your home country. That can be done by showing family ties, work ties etc. In the case of the 60 year old Indian grandmother who wants to see her children and grand children this could be very hard to proof.

We can now debate if we think the US prejudice that everyone wants to immigrate until he has proven otherwise is a good thing or not.

I think it has a good side. It makes it way easier to immigrate to the US and becoming an US citizen. A country like the US which takes a great benefit from highly qualified immigrants can attract a lot of people this way and can select the ones which fit the demands of the country. In the 18th and 19th century workers and farmers were needed and so a lot of poor Europeans came to the US. Now mainly highly qualified people are needed. The brain drain to the US is still high. Not as high as it was before 9/11 but still high. West European countries on the contrary especially Germany mainly allowed people in that seeked asylum because of political reasons or which fled from wars. Anyway it was until recently almost impossible for these people to become citizens because the idea was that these people will go back home as soon the situation in their home country gets better.

The disadvantage of the US system is that you as applicant have to proof that you don't want to immigrate. This is now even more complicated because of the paranoid atmosphere of the US government who sees behind every tree and rock a mean terrorist. So as an applicant you have to proof you don't want to immigrate and that you are harmless.