Besides the reasons already mentioned, the standard Hogwarts letter in canon had restrictions on what kinds of animals were allowed as pets -- owls (like Harry has), cats (like Hermione gets in PoA), and toads (like Neville's). All three are stereotypical wizardy/witchy beasts.
The Weasley rat is not explained in canon but can be assumed to have been granted as an exception. A snake would probably not be given such an exception, as it might be used to scare other children.
Or perhaps snakes are disallowed because then the Gryffindors would ask to be allowed pet lions.
Or perhaps snakes are disallowed because then the Gryffindors would ask to be allowed pet lions.
And you wouldn't want an explicit rule against pet lions, because then the Gryffindors would definitely have secret pet lions.
The new discussion thread (part 15) is here.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 82. The previous thread passed 1000 comments as of the time of this writing, and so has long passed 500. Comment in the 13th thread until you read chapter 82.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13.
As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: