I think you are thinking about it differently.
These companies rely on your promotion. So when the host says every week
tweet your questions and comments to
us.
They are asking for you to
1) Follow them / Place them a list - They want to know that you are looking at their timeline and that your followers see that you are following the company
2) Mention {@username} them / Reply. Here you are promoting communication to the Radio Show, depending on how their twitter client is set up, these "mentions" will be received by the user/company in a better format than
Science Friday Rocks
which then all tweets on Science Friday will need to be searched by hand. They could have used a #hashtag (e.g. #scifri instead ,thus clumping the event messages) to achieve the same objective.
On inspecting the site you have linked it seems directions are given as to what to do to ensure your message is received
Use Twitter? follow @scifri to get our updates, and tweet your questions and comments to us during the program.
I have hyperlinked the relevant portion of their message. So there are three ways I know that you could have found their account on twitter.
1) http://twitter.com/#!/@scifri
2) http://twitter.com/@scifri
3) http://twitter.com/scifri
Any would lead to their twitter account.
Now I do not know much about scifri but it does not seem like they acknowledge receipt of your mentions via their account. Maybe during their podcast (Fridays, 2-4 pm Eastern time), the interesting comments/questions are selected for the rest of the audience to see.
In your next example with support requests, I think you should look at
http://twitter.com/support
and
http://twitter.com/delbius
who are representatives for Twitter Support Team, as said on the support account page
Make sure you are following us if you'd like a response to your DM
I would hope that the company you are working with executes this in a similar manner
- Ensure whatever needs to be sent in private will be done as a direct message
- If the information can benefit the customers at large then it shall be broadcasted in public
Companies also tend to use Twitter to try to persuade users into writing shorter questions (getting content into 140 characters) so that they can read and respond as quickly as it was sent.