Insider trading is illegal, period. If the accused Senators traded based on insider, i.e. non-public, information, it's illegal. And courts have made the definition of "non-public information" fairly clear. So, if any of them traded based on non-public information, legal consequences should ensue, just as would happen to me if I traded in my employer's stock based on non-public information about my company. On the other hand, if they traded based on information from brokers or company press releases or other public sources, then what they did was both legal and ethical.
This is not a vague or fuzzy matter.