Why people are thinking that marketing means manipulation and what marketers have to do to change this myth?

Excellent observation, and in my view, it because marketing is sometimes confused with sales.

Marketing is about presenting information and ambiance about a product, to help it sell. 'Sales' is about closing the deal.

Also in markets where caveat emptor prevails - the buyer beware, being a bit more careful of 'sales' and 'marketing' tactics is a pretty good idea.

In my view, this is why Online Shopping is growing incrementally. Shoppers make decisions in neutral spaces.

the last step in the buying cycle is reevaluating the buying decision, this is normally accompanied by doubts of the buying decision after buying, this myth is originated from this step in the buying cycle. reassurance and after sale marketing activities is a good way to change this myth for our customers, also increasing  brand equity by good marketing activities will decrease the customer's doubts and fears and decrease his feeling of being manipulated.

ona very practical note, one cannot blame the people for having such a negative notion. we ourselves as consumers tend to have the same idea when  for example we are shown a delicious, sizable portion of a  food item in an adv, only to get disappointed when the actual item is presented.

The only way out is ethical which means show a quality product to the consumer and present the same. trust is the only way and big consumer names are thriving on it. yes sometimes they even tend to lower themselves to manipulation but far less.