Let's deconstruct a Congressional "earmark" inserted in favor of a constituent group.
An earmark is a mandated expenditure of public money for the specific benefit of known constituents of one or more Representatives or Senators (let's call them, "Congressmen", for short). For example, an article in The Wall Street Journal recently mentioned an earmark that an environmental group desired - namely a "bridge" for large animals to cross an Interstate highway.
So some Congressmen put an earmark into an appropriations bill directing public money to be spent for this project. The environmental group was very happy, so it was reported. Presumably they will support the Congressmen responsible, either with contributions or favorable publicity & comments or with campaign work, either for the Congressmen or against their opponents.
Does this seem reminiscent of a "bribe" ?
In hard sciences, sometimes one can determine information about the inherent mechanisms by looking at the inverse of a process.
Suppose a private person spends money to fix up a Congressman's country home. And in turn, the Congressman inserts favorable legislation into a bill that the person wants. Hmmm .... that's certainly a bribe.
So since the inverse process is a bribe, characterizing many or most "earmarks" as bribes seems very logical and correct.