Official Baseball Rule 6.02(c)(7), which MLB has been cracking down on since instituting inspections several years ago, pertains to pitching prohibitions and states, "The Pitcher shall not have on their person, or in their possession, any foreign substance." Reading on, we see this rule applies to both substances and objects that are "foreign"—not part of approved equipment or uniform apparel.
By ordering the towel removed—even though it appeared Jacob was simply using the towel to wipe his hands due to high humidity in Philadelphia—Bellino sought to prevent a future issue wherein the towel may have been used as a vehicle to transport foreign substances. Jacob had been wiping his hand on the towel after each of his first two pitches and in order to prevent a future issue, the towel was simply removed from play.
In order for the towel to be allowed, either a rules change or memo from the league office would have to be on the books. As it stands today, the towel is considered a foreign substance and thus not allowed.
Video as follows:
Alternate Link: Jacob's big league debut sees pocket towel removed from play
