Yes, a license is required, the licensee must pay federal excise tax and I would imagine the liability insurance is pretty high!!
http://www.atf.gov/firearms/faq/manufacturers.html
Q: Is a person who reloads ammunition required to be licensed as a manufacturer?
Yes, if the person engages in the business of selling or distributing reloads for the purpose of livelihood and profit. No, if the person reloads only for personal use.
[18 U.S.C. 922(a) (i) and 923(a), 27 CFR 478.41]
Q: Must a licensed manufacturer pay excise taxes?
Yes. Licensed manufacturers incur excise tax on the sale of firearms and ammunition manufactured. See Item 17, “Federal Excise Tax” in the General Information section of this publication.
I guess that explains why lol. I bet you're right that liability insurance is high. I checked out the link and it said that if you have a Firearms Manufacurer FFL, you can do ammunition without a seperate one too... that's pretty handy.