Snape's words in the top half of the page on zir exact words from Harry Potter and the Sorcerer's Stone, pages 136-138. This is one of 4-6 times (depending how strictly you count) when I will replicate a specific scene from the books.

There's no particular pattern to when I modify existing scenes and when I invent my own. For some of them, I use them to critique or comment on how the scene plays out in the books. Mostly, it's just a matter of which ones I came up with good uses for.

Snape could easily have blocked Harry from reading zir mind, but ze wasn't expecting a first-year student to have that power. In Snape's more advanced classes, ze is more on-guard against unscrupulous students who know Legilimency, but with first-years, ze needs all of zir concentration to keep them from getting themselves killed with potion-making mistakes.