Perhaps Thalys itself doesn't know which orientation the train will be.
This is the case with CrossCountry Trains in the UK, as their routes involve frequent direction reversals, and trains can go either way into depots, so they really don't know until the train actually starts its journey.
Indeed, I experienced that on CrossCountry last Saturday! My reserved seat was facing backwards; luckily, there were PLENTY of unreserved seats facing forward. Also, the train ride was short. The Thalys journey, however, will be 4 3/4 hours.