I have spent quite a bit of time in Latvia.
It is not a place known for scamming because few wife hunters venture there. Also, as an EU member and the second affluent of the Baltic countries, there isnt the level of desperation to fuel any scamming as there is in poorer places.
What do venture there are British and other guys on stag nights etc. on budget flights. This makes many of the women in and around Riga do this
at the mention of foreign men.
Anyone holding a Latvian passport should fall under the US visa waiver programme; allowing them to visit the US as a tourist with little formality. Not all residents will hold a Latvian passport. Some will hold Russian passports and some will hold grey alien passports.
On the whole, it is a place I think very worthwhile to look in. Partially because of the afore mentioned lack of desperation, and partly because it is off the well-worn route to Odessa and Kiev so many seem to want to tread.
If it were me, I would be looking outside Riga primarily. While you are looking there, extend your search to Lithuania and southern Estonia too.
I have no experience of the site you mention. There are an interesting mix of locations of women listed though; I just had a peek.