There is a lot of confusing information on the internet regarding this. Additionally, there have been changes made by the IRS later than many articles that someone may find by a web search.
I have not found any reliable legal source indicating that it is against the law for a plan to allow for an in-service rollover below 59-1/2, although many sources would claim it can only be done after 59-1/2, but they usually caviate this as "without incurring a tax penalty".
Clearly the industry has reason to steer companies to agree to plans restricting employee options and keep the funds where they are. It also seems typical that HR personnel do not usually have the expertise or a tax attorney or financial consultant.
In the early 2000's I worked for a company that would allow an in-service withdrawal, one time annually. As long as these funds were transfered to a qualifying IRA within the time allowed (60 days), there would be no tax or penalty.