In America it's more easier because the system of 'winner takes it all!' European nations are very small and voters are of more or less having same back ground. On the contrary, India is a hugely diversified nation. The language changes in after every fifty kilometer. In one square kilometer of urban area one would find number of religions, cultures, ideologies etc. It's a vast country.
Thus it's always difficult to predict anything with a sample size of some thousand votes. Thus whenever opinion polls come people very well know that these are speculations, but still wanted to know what that sample actually says.
Here one thing must be noted. Despite small sample size it can nearly ascertain what would be probable vote percentage going to parties. The real problem is with converting vote percentage into seats. There is absolutely no formula. Whatever methods are used is just based on past history, recent results, condition of contest (such as bi-polar or multi polar etc) and presence of particular party (whether it's presence confined to a specific part or spread over entire area). One can say that converting vote share into seat projection is like practicing astrology with an error margin.
CSDS and AC Neilson didn't agree to manipulate the projection during the sting operation. But their poll projection too goes wrong many a time. Even C-Voter although agreed to do manipulation, but it was not ready to do it in its name, rather with its sister organization through a colleague. That means though there are some corrupt people in the trade, but there are some good and honest agencies too. Their method may be at an early stage for a country like India, but regular practice would make them better.
But question is, in a democracy, why the ban word always comes out? Those who had taken black-money and exposed, police action should be taken against them. At the same time opinion polls should be allowed so as to prove their credibility.
The Congress claims UPA would return to the power. NDA too claims that they will form next government. Modi's followers say that BJP will cross the magic figure of 272. Technically all these are opinions. Such opinions were always claimed by the parties in last many elections.
Now will you ban political parties from claiming that they would win or not? Arvind Kejriwal immediately jumped his gun and said that BJP is giving money to manipulate the opinion polls. He perhaps forgot that AAP had done an opinion poll, citing to win 2/3rd majority in the election. The result of this opinion poll was pasted on the back of most auto-rickshaws! If one complains that the AAP's opinion poll was manipulated, what would be his answer?
Everybody has a right to give an opinion. One can also have a right to propagate his assessment of survey! Those who do unethical let them be booked. But banning opinion polls would be undemocratic. This would mean that public is fool. They may be influenced wrongly. This idea is just an insult to the master of the democracy.
Please note, in 2004 each and every opinion poll, even exit polls projected NDA's return. But people voted otherwise. Similarly in 2009 at least 50% opinion polls projected Advani as the Prime Minister, but again People voted otherwise. Thus none should question public's intelligence. Also there should be no ban on opinion poll or opinion making.