I'm at something of a loss to know exactly what the TSA memo means by "controversial opinions" but I can certainly understand their "groping" for a term to express an objectionable misuse of "company computers."
Generally, if an employee wastes time at a porn site, only that employee is wasting time.
When one or two people within an organization frequent "slanted opinion" sites, where the opinions are controversial among a significant portion of those and other workers, the "debates" that inevitably follow are a disturbance to the entire office, and can be much more disruptive than other "diversions."
If those who visit a site don't choose to preach and argue about it in the office then it's NOT a controversial site, if we - the uninformed - a permitted to guess.
My suspicion is that there's been too much "politicking" - and/or "preaching" in (some?) TSA offices, and this reference is just an overly polite(?) way of saying get the debates out to the barroom on your own time.
There are times when one needs to be a bit obscure in order to avoid being brutally blunt. Being too explicit offends (and often identifies) the people whose "attitudes" need "adjusting," neither of which is a particularly productive management effect.