Shouldn't everyone get at least 100 rep when registration to this community? Too many down vote leads to lowest rep? But why there are so many of such users? Should we be alarmed by this phenomenon?
All normal users start on +1 rep.
If you check out the What is reputation? How do I earn (and lose) it? page you'll notice:
- site association bonus: +100 on each site (awarded a maximum of one time per site)
It goes on to explain this bonus:
If you are an experienced Stack Exchange network user with 200 or more reputation on at least one site, you will receive a starting +100 reputation bonus to get you past basic new user restrictions. This will happen automatically on all current Stack Exchange sites where you have an account, and on any other Stack Exchange sites at the time you log in.
The +100 rep is from the site association bonus. Association bonus is triggered if you have more than 200 rep on any SE site. If a user doesn't have 200+ rep on any SE site they will start at +1 rep.