I know. Same here. It may be intentional blocking, but most likely and realistically is just crappy and buggy reservations systems using cached/outdated data. Unless you try to ticket it, you can't really know if what you are seeing is really available and bookable or not. (Happens to all airlines. AA sometimes shows AY flights phantom availability also. Other times it doesn't show AY flights online, but if you see it online on BA.com and call to book then it is available and you can still book it. Here I mean, unless you call DL and try to get a competent agent to really try to search for and book the SU flights you are not seeing on delta.com, you can't really know are they really not available for booking, or just not showing up in standard (buggy) searches.)

For all that is holy and good in the universe I certainly hope that DL/SkyTeam are not moving to a revenue-based award pricing model across multiple airlines (i.e. SkyTeam/partner) awards. I am hoping and praying that the sheer complexity of implementing that is past the competence levels of DL IT (or any airline/alliance, for that matter).