Having the opinion that gambling can destroy our finances, can lead to gambling addiction etc. is completely fine. I'm not denying that gambling can have negative side-effects. But many activities can have side-effects, impact our mental health and our finances negatively. So, my point of view is: let everyone decide if they want to gamble or not.
And yes, opposing gambling is a legitimate opinion, definitely.
The whole issue here is BenCodie's participation in a paid signature campaign of a gambling service and opposing gambling heavily at the same time. 
While he's calling gambling "unethical" etc. he's getting paid to advertise it via his signature at the same time, which will cause more people to sign up for it.
Like Rikafip said, this is undermining his credibility to advocate against gambling and BenCodie is actively giving views for the gambling site via his paid signature because that's what wearing a signature does.

---

The only one who needs to be embarrassed is BenCodie, who's getting paid from a gambling site, while he believes, Gambling is unethical. 
It's not credible at all to oppose gambling on one hand and on the other hand, to wear a paid signature of a gambling site.