Because it's a universal offer and not aimed at anyone with specific devices. It is a good offer and if I am honest the NOW stick is veery poor compared to Roku and Amazon, which are very cheap.
An Amazon Fire Stick should be fine.
On the NOW Community Forum, you find some members prefer an Amazon Fire Stick and some prefer a Roku player.
I own both and really haven't got a preferred favourite where they both great TV streamers for me.
I have cancelled my NOW Cinema Membership this week and the Cinema Membership has now expired, on the hope of getting this 6 months Paramount + offer (so far nothing for me with regards to Paramount + 😫).
Ouch bud for the cinema cancellation. Really hope it wasn't a limited time offer 😐
Same as @schnapps where I have owned an Amazon Fire stick and Roku stick. Both are very good for different reasons.
Amazon fire sticks allow you to connect via Ethernet through an adapter. However the UI for the fire stick is awful, far too cluttered.
Roku stick has a much easier UI and all the apps on the homepage unlike Amazon.
For me I have entered the Apple TV neighbourhood since I have an iPhone and iPad. Even though it's very expensive it is by far the fastest device I have ever owned and much like Roku a nice clean UI with the apps on the homepage.
More importantly for me the use of VPNs are coming to Apple TV, whereas Roku you can't and Amazon you can.
It will certainly be down to personal preference. One thing I could say is the homage of Roku will be very similar to the NOW devices.