From the words of the Author
George R.R. Martin says in an e-mail correspondence:
[Interviewer] Yet if every knight can create a knight as we have seen in "The Hedge Knight", what prevents a widespread misuse by unscrupulous hedge knights such as Ser Osmynd Kettleblack or the cynical Great Houses?
[GRRM] Social pressure. A knight's peers would look with a certain amount of disfavor on anyone who did this. They might gain money, but they would lose honor. And honor is still very important in this culture.
There's more in the interview on the subject, but this is the core of it.
Repute
We know that any knight can make a knight1, but to make it 'stick' the knighting knight needs to have the appropriate renown, as explained by the greatest knight of all, [Ser Barristan "The Bold" Selmy][1]Ser Barristan "The Bold" Selmy:
- [Ser Duncan The Tall][2]Ser Duncan The Tall is featured in the novellas The Dunk and Egg Stories. Dunk himself has difficulty proving his knighthood, but in the end he got vouched for by [Prince Baelor "Breakspear" Targaryen][3]Prince Baelor "Breakspear" Targaryen.
- Also from The Dunk and Egg Stories (The Mystery Knight), [Ser Glendon Ball][4]Ser Glendon Ball claims that he was knighted in front of many witnesses2.
**3.** From the [wiki][5][wiki][6]:
> Knights are supposed to be brave, courageous, honorable, and true to their word. They should be loyal to their feudal overlord, and are tasked with defending their faith. In practice, most knights fall short of sustaining such high and noble ideals. Many do not try particularly hard, and instead seek knighthood mainly for the prestige and opportunities.
**4** See this conversation between Dunk and Egg:
>"Do you want a clout in the ear? What book is that?" Dunk saw bright colors on the page, little painted shields hiding in amongst the letters.
"A roll of arms, ser."
"Looking for the Fiddler? You won't find him. They don't put hedge knights in those rolls, just lords and champions."
*-Dunk and Egg Stories, The Mystery Knight*
[1]: http://awoiaf.westeros.org/index.php/Barristan_Selmy[2]: http://awoiaf.westeros.org/index.php/Dunk[3]: http://awoiaf.westeros.org/index.php/Baelor_Targaryen[4]: http://awoiaf.westeros.org/index.php/Glendon_Flowers[5]: http://awoiaf.westeros.org/index.php/Knight