During Queues, Team Builder does specify the position in which you want to play, but with bot lane, two supports (ex. Nami and Thresh) can both be added to the same queue, which means that no adc (marksman) is chosen. I do believe that it needs to be a little more specific with the bot lane.
Not to be rude, but shouldn't you have posted this in the pbe forums?
Also, this could be because the Captain can only choose where he/she wants people, but not not what roles he/she wants in those positions. If the Captain wants 2 people bot lane it just searches for 2 people in bot lane, not specific roles/champions. I agree that this should be fixed. I actually brought it up the idea of being able to search for roles as well as positions in this thread: http://community.pbe.leagueoflegends.com/c/gameplay/2x1YOG56-teambuilder-social-experiment-and-breaking-the-system-new-metas?comment=0007
