Tadpole motortrikes.

[reohn2]

Further back in this thread you will find that these specific machines have no lock. Some scooters do however. I'd say it is simply a matter of sponsorship -free supply of machines.

Sorry missed that
As you were

Though even without a lockout I imagine a three wheel platform is quite possibly easier at (very) low speed than a two: you can use your arms to keep it upright and straight, while on a bike you need to steer to keep it upright.

They could have a progressive lean feature where the slower you go the more the bike relies on steering rather than leaning.

[mattheus]

I'd assumed that an advantage relevant to a TV moto was being more stable as a camera platform when going v-e-r-y slowly matching riders up very steep climbs through throngs of cheering fans.

That seems the most convincing justification so far!

(although I've never ridden a trike more than 20metres, and never ridden a powered 2-wheeler ... )