The Southern Baptist Convention in the Public Sphere
There is no evidence that the SBC believes itself to be barred from issues relating to public policy, and there is much evidence to the contrary. First, let's look at the SBC's understanding of the Resolutions it issues (emphasis mine):
[...] Covering a wide range of theological, social, and practical topics,
resolutions educate our own people about important moral, ethical, and
public policy issues; speak to the broader culture about our beliefs;
and provide helpful tools for our churches and entities to speak with
authority in the public square about the biblical application of
timely and timeless matters. [...]
-Southern Baptist Convention: A Closer Look
Other Examples of the SBC opining on Public Policy
Let me give four easy examples of official SBC resolutions which relate to public policy and were given in the last three years:
- Resolution on Immigration (2018)
- Resolution on Defunding and Investigating Planned Parenthood (2017)
- Resolution on Freedom of the Press (2016)
- Resolution on Women Registering for the Draft (2016)
Concluding Answer
Paul Chitwood believes that the legalization of marijuana will have a detrimental effect on the well being of children. He therefore decided to publicize and garner support for this issue within the churches of the SBC. His decision to involve the SBC in a question of public policy is consistent with the stated principles of the SBC and is also consistent with SBC precedent.
(While I acknowledge that this answer is not Biblical, it does draw on official SBC sources as requested by the original questioner's bounty description, "Looking for an answer drawing from credible and/or official sources." Given the theocratic nature of all cultures found within the Bible, seeking Biblical support on this issue may well be an anachronistic endeavor.)