You really opened a can of worms here by suggesting to the public to use a particular Sponsorer. And you made it worse by comparing this site to NPR.
First of all NPR is a non-profit so their whole working is based on donations and sponsorship. You are not. You are a For-Profit organization looking to make money with this site.
No Problem… Kudos to you for building this site. And I hope you are successful.
What is not right is your personal recommendation for a service provider. Even if you disclose that you are doing it because they are sponsoring this site it is still Unprofessional.
NPR does not say, please buy or use a particular vendor. They say their name, they advertise and they get paid for it. But I have never heard anyone say,
‘He is very professional and very nice person. Use him….’
Nope. Your Avatar says that you are a moderator, so moderate.
As you said this is a public forum… You might think you’re first responsibility is to the paying few however I think a better business model is to serve the public. They are your real customers. I understand that you want Sponsors, and you will get them. Let the site grow without forcing it… As it grows, people will come to advertise.