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.