I first saw a thread about this topic on NB's forum last week. I didn't weigh in there, so I'll add my $.02 here.
First, I don't actively practice business law (I work in the public sector), but I know a thing or two about trademarks.
Second, trademark claims fall into one of two categories: 1) actions based on "confusion"; and 2) actions based on "dilution." Regarding confusion, the issue here is whether consumers are likely to be confused as to whether there is a brand association, endorsement, etc., between the protected mark ("Two Hearted") and the allegedly infringing mark ("Three Hearted"). In the case of NB's Three-Hearted Kit, I think Bell's has a legitimate concern with the possibility of consumers believing that Bell's might have some sort of affiliation with the NB product, especially given NB's recent introduction of the "Pro Series" kits, where actual breweries have formulated the clone recipes. In fact, it may not be coincidence that the issue with Bell's only arose after
NB started offering the Pro Series kits. In any case, there are several factors that a court looks to in determining whether consumers are likely to be confused (e.g., the similarity of the marks, the relevant market, etc.). And since trademark actions are civil matters, the burden of proof is a preponderance of the evidence (i.e., more likely than not). So the ultimate question is whether it is more likely than not that a consumer would conflate in some way Bell's Two Hearted with the Three Hearted Ale kit. You
make the call.
Third, regarding dilution, the issue here is whether the allegedly infringing mark somehow dilutes the protected mark. The question to ask is whether the uniqueness of the protected mark is somehow diluted, i.e., "cheapened," by the allegedly infringing mark. It would be kind of cold for Bell's to argue this, since so many homebrewers think very positively of Bell's beer, but again, I can envision Bell's argument: the NB product aimed at homebrewers dilutes the brand because the authentic commercial product is guaranteed to taste exactly how it should and the homebrew kit may or may not taste how it should based on the skill of, and the process used by, a homebrewer. The potential for the kit to vary in quality and taste from the actual product dilutes the brand.
Finally, as to the propriety of Larry Bell taking this sort of action, I'm not sure I feel that great about it. I know they're just defending their brand, but a lot of commercial brewers I know seem more likely to take a "live and let live" attitude toward something like this. At the very least, I think Bell's could have had a win-win if they had been willing to come to the table with NB and design a Pro Series Two Hearted Kit.