Why did the writers / producers of the West Wing spoil the final season in episode 1?

As a huge The West Wing fan, to me the flash-forward was a pleasant surprise from the regular chronological ⁽¹⁾ story telling, and I did not experience it as being a spoiler. In fact, as trigger to keep watching, quite the opposite:

• At the end of the sixth season, it is unclear if Toby will be caught for leaking classified information, so Bartlet to Toby saying "Glad you could make it." could mean anything from Toby being free and working abroad to Toby gotten a temporary release from prison.
• For six seasons long already, we are in anticipation of a growing relationship between CJ and Danny. Seeing them together in that scene answers that indeed, but it was not within expectancy anymore because Danny's last appearance was in the first episode of the fifth season, two years ago. So, now we are told they will end up together. But it actually raises the question of how they will, since in all previous episodes they were together, it did not last. (By the way, their reconciliation is worked out superbly at the end of the season.)
• Will's "future" being revealed really is not that spoiling kind of exposure.

• The scene clearly does not reveal who the (next) President is. Or better: clearly intends not to reveal. We see his shoes getting out of a car and we see Bartlet shaking hands with... another hand, while the camera pans up till just not showing his face.

  Josh being there introducing the President could indeed mean Santos won, but Josh could as well be in a ceremonial function. It could also mean he works for Vinick. It is a long shot, but in three years a lot of things can happen. His patriotism could have made convince him as democrat working for a republican White House. After all, it was Josh who came with the idea to appoint republican Christopher Mulready in The Supremes.

  Also, at the time this episode aired, it was not settled Santos would win the election. That was only after John Spencer's death. From 'West Wing' Writers' Novel Way of Picking the President:

  Instead, Lawrence O'Donnell, an executive producer of the show, said he and his fellow writers had declared Santos the winner only after the death, in mid-December, of John Spencer, who portrayed Santos's running mate, Leo McGarry. At the time of Mr. Spencer's death, the plot for last night's episode had been set: the election was to be won by Alan Alda's Arnold Vinick, a maverick Republican (modeled a bit on Senator John McCain), whom many Democrats (including the Democrats who write the show) could learn to love.

⁽¹⁾ There are flash-backs in Two Cathedrals and in An Khe. There is also a flash-forward in Inauguration, Part I.

It seems Santos' victory was planned from the start:

John Wells noted that the Santos victory episode had already been written.

According to O’Donnell, the situation was more a matter of the writers still arguing for Vinick but the death of Spencer silenced them and confirmed that Santos would win. As O’Donnell later noted in a BBC documentary on fictional presidents:

We actually planned at the outset for Jimmy Smits to win, that was our .. just .. plan of how this was all going to work, but the Vinick character came on so strong in the show, and was so effective, it became a real contest … and it became a real contest in the West Wing writer’s room.

So I imagine that that is what O’Donnell was talking about in the New York Times article. That Vinick winning the presidency was gaining steam but Spencer’s death squelched it.

There was no indication Josh was working for the President - Josh could have just been told by someone that the President was arriving and was relaying the news to those inside. People are getting confused because they're applying knowledge learned later on to the scene.