There are thousands of Roku apps; creating one is not a high-tech challenge. This is just HBO [edit, thanks sibling comment by 'whatok [0]:] refusing to pay Roku's toll.
Not having a roku app is a deliberate choice. It's not like this is some engineering failure. Hbo and roku got in some sort of fight, I assume hbo wants to be paid by roku.
HBO isn't paying WB, HBO is WB.
Whoops, that's a good point. I guess the accounting works a bit differently in that case?