There are plenty of tickets sold that aren't by season ticket holders. And While someone might be willing and able to spend $70 to go to a game every week, doesn't mean it's wrong of them to sell one of those tickets for $400 if they can.
If I lived in Dallas I'd get season tickets, and plan to go to every game, but if along the way there was a game like against the Pats where tickets were going extremely high, like $500 or more, then unless I'm really well off, it'd be kind of stupid not to sell it. I would not pay $500 a ticket to see a game, and if that's how much they're going for, then by not selling it, that's pretty much what I'd be doing.
A lot of people like myself would be able to afford the season ticket prices, but are still not wealthy enough that an extra $500 or so dollars wouldn't be beneficial. If given the choice of 9 games (2 preseaon) at $200 or all 10 games at $700, it seems to me a no brainer. I could sacrafice seeing a game in person for $500.
And sometimes emergencies come up in people's lives where they could use a little extra $$. Life doesn't always go as planned.