Thats because plant patents cover varieties that won't really propagated themselves asexually naturally (or won't come true from seed). Tubers will keep propagating by themselves - you cant call out someone for infringing a patent on a potato if you miss one when harvesting and it makes more of itself. Although there's always grey area - plant reproduction is complicated! Tubers and seeds are however covered under plant variety protection, but that's a different legal code than patents.

Why incur the cost of printing this on the package? It’s a trivial expense. But agricultural margins aren’t that wide.

Because some goof lawyer suggested it and the marginal cost of adding the text was ~0. A patent like that is a ticket to huge margins as far as agriculture is concerned. It makes sense that people would err on the side of protecting it.

Marketing perhaps? Maybe they think the notice makes the product seem exclusive and so high quality that it needs special government protection because everyone is dying to reproduce it.