Of course a game's sales aren't defined by its quality, and a game's quality isn't defined by its sales. That doesn't make it illegimate to be interested in the sales regardless. For some people, the data analysis is just genuinely interesting. Some people won't "get" that, and that's fine, but it'd be better for these people to just avoid Sales-Age topics rather than questioning why people could possibly be interested.
It's not fanboy wanking. It's just a matter of enjoying looking at and thinking about data. Plenty of people do this with sports, in comparing team rankings and game history and player stats; plenty of people do it with politics in analyzing election data; it's the same thing with games. For people with a certain type of personality, that kind of data analysis is just enjoyable. It can be fun to look at the way sales trends are going -- whether across a particular genre, or a series, or how things are selling on a system or from a particular company. And it can be fun to take those trends and then try to guess how something will sell in the future. Or to try and predict what will get made. How are RPGs selling on the DS in direct comparison to the GBA, which will determine future development? That's interesting! Or you can comment, "Oh, X game sold more than Z game, I wonder why," and then try to come up with the factors that could've caused such an outcome (or just appreciate the randomness of it all). And yeah, sometimes the desire for a particular sequel factors in -- othertimes not, and you're just looking at the data from a neutral, observational perspective because the numbers themselves and the meaning behind them is interesting enough alone.
It's difficult to explain properly, because you either get why this kind of information collecting and examining is enjoyable or you don't, but it's definitely annoying when people who don't enjoy it constantly question the validity of doing it at all.
(The above isn't solely directed at Tsu and Ami, it's just an annoyance that's been growing as people complain about this stuff consistently in every sales thread, and I've been meaning to express that annoyance).