Leech, I understand your concern. Cable and satellite providers are rolling out more and more "TV Everywhere" partnerships with programming providers all the time, so you expect that all online content should be covered by subscribing to the corresponding cable service. However, if you look at just who is providing this access, it is large programming groups, such as Turner Networks, Viacom and HBO Networks, to name a few. Smaller programmers may not have the resources to provide this expanded content.
The Anime Network is in the latter group. While it is the same programmer providing the VOD and online content, it does not have the wide "penetration" that networks like HBO, Cartoon Network and Nickelodeon have, nor does it sell advertising placed within the programs that airs on VOD. They even had to partner with Hulu to provide the streaming for guests and registered members, so the revenue from the advertising you see there goes to Hulu, not TAN. That is the plight of "niche" channels such as TAN. (Oh, and as far as I know, not even Funimation provides "TV Everywhere" access to its online content.)
If you want to support having anime available bot online and from your cable subscriber, I feel that it is well worth the cost to subscribe to TAN online even though you also subscribe to Anime Network On Demand through your cable provider. If you don't think supporting anime is worth the extra expense, don't subscribe-but don't complain if you find less and less anime available here in the US. You gave your vote by your actions.