Vernon,
I also have difficulty in understanding the sole use of Nitrite in the USA.
However I think that this paragraph in the link posted by Dave may be one of the reasons:
Effective June 15, 1978, the USDA changed the curing procedures of "pumped" bacon as follows: the use of sodium nitrate and potassium nitrate is prohibited; the level of ingoing sodium nitrite shall be 120 ppm (or 148 ppm potassium nitrite); the level of ingoing sodium ascorbate (vitamin C) or sodium erythorbate (isoascorbate) shall be 550 ppm. According to USDA surveys, these changes have resulted in bacon that does not form nitrosamines when cooked at 340 degrees F for 3 minutes on each side. These three changes apply only to pumped bacon and do not apply to dry cured bacon.
The US science guys have obviously tested the products cured in this way and found them OK, therefore the rule?
Regarding your reply to Lance re Curing without Nitrite/Nitrate you, rightly, say that botulism is anerobic and go on to say that the bacon can be smoked - wouldn't the be conditions be anerobic whilst being smoked?
Congratulations on what you are doing with the products available to you. I was amazed by the colour of your ham given no nitrite/nitrate.
Personally, as it is available to me, I will use Nitrite in 'short' cures and a combination of nitrite/nitrate in 'longer' cures and smoked/dried products because of the added protection they provide. I choose to cure within both the European and American levels as they are the only scientifically researched standards I can access.
There seems to be no reason for you not to use nitrite/nitrate in your hams, when you can get hold of some, because as far as I can tell, Nitrosamines are only a problem with high temperature cooking such as with fried bacon.
Regards
Phil