As a general rule, it is easier to go from an odd amount to an an even amount, not vice versa. Disney will not pull up an extra chair for an extra member, because there just isn't room. If someone has a wheelchair or stroller, they take a chair away, not stick the wheelchair on the side. Also, there aren't many booths, because they allow for less flexibility in table configurations.
If they can accommodate you by sitting you at a table for 6, they will. But don't count on it, especially at popular restaurants. They are often full to capacity from open to close.
Also, keep in mind it isn't just Disney being greedy or a jerk. There are fire codes they have to abide by. And they are trying to keep guests and castmembers safe from accidents by allowing enough room for everyone to maneuver through the restaurant safely and efficiently.