Maybe it's legal, but it should absolutely not be. There's too much risk that a legislator's personal interest will run counter to the interests of the public, and the system should have safeguards against this.
Unfortunately that risk is very much in play even without stock trading. Politicians often appear to be working purely to stay in office and gain whatever side benefits they can along the way, and to establish a very lucrative lobby landing after they leave office.
It is all practically very corrupt, viewed from most angles. The few well-meaning politicians either don't get elected or don't stay in office, because it costs too much to get there and stay there (funding); so to get there and stay there, you have to play the game. And playing the game means ensuring your wealthy donors get what they want from you in terms of legislation.
Yeah that's all fine, but you can still work to outlaw the most blatant conflicts of interest. Government isn't perfect anywhere, but for instance in northern europe they're doing at least a tiny bit better on that front.
