Weaponacf-
The political agenda you cite is the agenda of the people/groups that PURCHASE THE AD SPACE. For the most part, Roku is just transmitting the program and ads as received from the program source, so Roku is no more responsible for the ad choice than the internet service you use to bring in the signal, or the TV you use to display it.
Ads are purchased and inserted both nationally, and regionally. If you live in an area that one party or the other thinks they need to really "push", you're likely to see a preponderance of that party's advertising. Likewise if you live in an area that one party thinks they "have in the bag", you may see little or none of their advertising. And then there's the matter of which party has more money to spend.