Trump should think about the ramifications of his actions -- the downsides to some of the people he is supposed to represent.
But Trump doesn't think. He shoots from the lip and makes decisions based on his mood at a certain moment.
The government should not take the stock markets into account when making decisions. Investors should take government into account when making theirs. It would be unwise and unfair to base policy decisions on protecting current gamblers.
Besides, if the markets go up or the markets go down, the net change is nothing. Nothing is created or destroyed. Some buyers get a better/worse deal than they would have if nothing had changed, some sellers are in the opposite position.