Picking up a fresh, operational, unaltered grenade, under controlled or semi-controlled circumstances, will usually not make it go boom.

However, grenades carried by civilians into situations like commercial aviation are typically not fresh-from-the-factory ordnance. Rather, they are usually war souvenirs, which can be quite old and no longer be in nominal operating condition. The pin could be rusted, the spoon could be loose, core could be unstable, the explosive load could be contaminated or deteriorated or unstable. Any such instability increases the risk of the weapon going boom from casual handling.

Likewise, there is no way to tell prior to picking it up whether such a weapon is in original configuration or has been altered in some fashion, such as inclusion in an IED, and any such alteration can de-stabilize the weapon, again, increasing the risk of it going boom from casual handling.

In this particular case, we're talking about a grenade. Though it could be a live grenade, an inert grenade, or a realistic replica, it is simple prudence to treat it as a real, live grenade until proven otherwise.

Though it could be a stand-alone device carried by someone meaning no harm, or it could be a component of an IED intended to do harm, it is simple prudence to treat it as part of an IED - thus altered, unstable, and potentially booby-trapped - until proven otherwise.

Basically, if you see a grenade, treat it like live wire: DON'T FREAKIN' TOUCH IT.