Who says he eats any mushrooms, including the Magic Mushrooms?
From page 8 of the SMB1 manual: Thanks to “the Koopa, a tribe of turtles famous for their black magic”... “Mushroom People were turned into mere stones”... “The only one who can undo the magic spell is Princess Toadstool”.
A section of SMB1's manual shows some sprites, including page 14 which shows the “Mushroom retainers”, which are the people who look like Toad found at the end of the castles. The manual says “Seven Mushrooms who originally served in the court of Princess Toadstool”. In other words, the term “mushroom” can be used to refer to a person who is a citizen of the Mushroom Kingdom.
In the section called “Mario's Friends”, which discusses the Magic Mushroom, Fireflower, and Starman, page 8 of the manual says, “If you come
across mushrooms who have been turned into bricks or made invisible, they reward you by giving you a power boost.”
So although Princess Toadstool may be the only one who can take care of the magic spell over the whole land, Mario is able to rescue individual friends by shaking them up a bit. Surely we don't usually think that Mario is eating the Fireflower. Simply making contact with the Power-Up is sufficient. (This is clearly shown by the Super Mario Bros. Super Show cartoon's intro, when Mario touches a Starman and that causes him to don the Fiery Mario clothing seen on SMB1's box art, since we all know that's exactly how that works). Likewise, several pieces of official artwork show Mario holding a Magic Mushroom, but not eating one.
If Mario was known to eat his friends, they would be far less likely to be providing him with rewards.