Whether it's an individual or a company, you can't buy shares...

Whether it's an individual or a company, you can't buy shares with insider knowledge and not be at risk of insider trading. Think about it like this - a company knows it's results are great and so keeps buying shares and the share price ends up popping - the individuals who sold at the lower price without that insider knowledge were unfairly disadvantaged. In the reverse, the company would not want to continue buying before reporting if it knew the result was terrible and the price was going to fall. They would suspend the buyback if they were rational and acting in the interests of shareholders (ie not wasting money).

Thus they have a blackout so that there is no chance for insider trading, and also no signalling as to the result.