Wikipedia has the answer to your question
"As my wish was to serve an oppressed people, and assist in a just and good cause, I conceived that the honor of it would be promoted by my declining to make even the usual profits of an author."
Wikipedia which cites Craig Nelson's book on Thomas Paine.
This is confirmed in multiple sites
Wired is a bit more expansive on the same quote
Were Paine to enjoy in 1995 the kind of literary success he had in his day, he would earn millions in royalties, rights, and speaking fees. But Paine didn't earn a shilling from the book. He paid the cost of publication for one edition - 30 pounds - himself, then donated the copyright and all royalties to the colonists' struggle for independence. Royalties would make his work more expensive, he feared, and thus less accessible. It's tough to imagine Paine's words coming out of some Washington journalist's mouth today: "As my wish was to serve an oppressed people, and assist in a just and good cause, I conceived that the honor of it would be promoted by my declining to make even the usual profits of an author, by the publication (of Common Sense) ... and there I gave up the profits of the first edition" - to be disposed of, he stipulated, "in any public service or private charity." This idea cost him, in the most literal sense: Paine was impoverished for much of his life.
Update: I realized I didn't explicitly address all of the questions. He did foregoe the royalties, and as far as I can tell he did so voluntarily. I'm not aware of any evidence that he did so in an attempt to increase the circulation of his writing, but I can't imagine that he would be unaware of the possibility.