due diligence for ulterior motives possible?

You mustn't assume that the business world is inherently immoral/unethical. If any co were to do this, they could do it only once because thereafter they would be known to be dishonest(!) In a DD scenario there is "give and take" - it's not all one sided - the target company has to be interested in being acquired and will do it's own research into the potential acquirer - and there will be "memorandum of understanding" or other agreement between the parties that will bind them regarding confidentiality.