Limit Loads
 

Slightly curious, why are most airliners certified to +2.5/-1.0 g while Normal Category pistons are usually +3.8/-1.9 g?

There is a lot more mass in an airliner which would require a lot more structure to to increase the load limit.

You planning on some aerobatics in a 737?

No but normal category aircraft can't be used for aerobatics and are still certified to higher.

Normal category aircraft are generally flown by private pilots who have (perhaps) little appreciation for load limits, whereas...large transport aircraft are flown by professional pilots....who have leaned (hopefully:ooh:) how to operate the airplane properly.

l would suggest that the weight/mass of the several tonnes of fuel lurking in the longer wings may have a part to play in that restriction.
Mind, a perfectly executed barrel roll should be ok, l`m told.

The loads on the airframe is a product of the accelleration (g load) and the mass of the airplane; that means, for the same build strength, with a lower design load limit expressed in G, the allowable mass is higher. That means you can put more useful stuff, like freight, people and fuel in the plane, without making it stronger. It's simply not nescessary to build air transport planes for higher loads, since building them is always a struggle between strength and weight - a stronger design is normally heavier, which in turns eats away the benefit. It's a delicate compromise.