Maybe, maybe not. I don't know what Enron's policies were, but it is increasingly common for companies that subsidize 401(k) plans to REQUIRE that a significant portion of the money be invested in the stock of their own companies. This is an artificial way of bolstering stock prices, and what happened at Enron may be an example of why this is such an intellectually (and morally) bankrupt idea.
While it may be acceptable for the company to dictate the terms of the company match, this sounds blatantly illegal if it involves the employee's 401(k) contributions. If it is only the company's matching funds that were lost, then the employees really have nothing to complain about since it wasn't their money to begin with.