Martin Maldonado had the following to say regarding his illegal bat:

A player may use a maple bat, but it must adhere to MLB regulations for, among other things, the slope of grain, according to Joel Sherman of the New York Post.

Since Pujols made his debut in 2001, well before the rule took effect, and was already familiar with his bats, the bat was grandfathered in.

The restriction is in place as a safety requirement, even though the league apparently could not perceive any performance benefits to utilizing the bat. 

MLB views that brand of bat as being riskier since it has a higher propensity to splinter into many pieces, according to Sherman.

The majority of players probably aren't aware of it till it's broken, as in Maldonado's case, because it's such a minor regulation. 

After all, Maldonado had seen his ex-teammate play his entire career using the same bats without breaking any regulations.