Comcast is confusing not returning modems with the crime of stealing modems. Stealing only occurs when someone gains possession of the item through illegal means. If possession of the item was rendered by the owner (i.e. Comcast gave the modem to a customer for use), failure to return creates civil liability--not criminal liability. It's the same reason people who don't pay their car loans aren't thieves.
What's worse, for Comcast to allege any one person has stolen a modem, or possesses a stolen modem--both crimes in every jurisdiction in America--without evidence or probable cause to believe a crime has occurred, is in itself libel.
Failure to return leased or rented equipment is indeed a crime in most if not all states. It's typically known as "Theft of rental property".
A quick search of "Theft of rental property, Pennsylvania" Turns up:
(a) Offense defined.--A person who obtains personal property under an agreement for the lease or rental of the property is guilty of theft if he intentionally deals with the property as his own.
(b) Definition.--As used in this section:
(1) A person “deals with the property as his own” if he sells, secretes, destroys, converts to his own use or otherwise disposes of the property.
-Title 18 Pa.C.S.A. Crimes and Offenses § 3932