Gee. Why is that? Background:

The professors reviewed more than 16,000 common stock transactions carried out by about 300 House members as revealed in the members’ financial-disclosure forms from 1985 to 2001.

Here. Two clarifications: 1) all corporate employees are restrained by insider trading laws, not just executives. It isn’t legal for non-executives to trade on insider information. 2) These laws are not limited to Wall Street. Again, the article is not clear. It isn’t legal for employees in industries other than financial services to trade on insider information.