How come Aniplex USA and Ponycanyon USA anime are sold exclusively via the RightStuf.com
Also, how come Ponycanyon USA seems to operate exactly like Aniplex USA, including its website layout?
I'm not sure what title you're referring to, but I did a quick Amazon search for Asterisk war limited edition and it came up without any issue. I had thought that Animate USA was also a vendor for Aniplex USA, but apparently they closed that down last year.
To provide a simple answer to your question: It's the Japanese style of business. Rather than to use complex distribution chains as is the standard in the US, vendors become approved sellers by making agreements with the original producer. It doesn't have much effect on price, but does retain a higher profit margin by doing in-house distribution (cutting out the middle man). An item bought from a US company will generally pass through a minimum of 3 separate distribution companies before finally making it to a retail location, and each of those are increasing costs in order to cover their overhead and profit. By doing in-house distribution, and limiting the number of vendors being sold to, business models such as print on demand and simplified shipping greatly decrease overhead and allow for those profits which would normally be taken up by the distribution chain to remain with the primary producer.
As to the website, I'm going to guess that both have opted to use a standardized tool for development and not really interested in spending a lot of money to make it fancy. The reality is they have a pretty secure lock on their audience because of being the license holders for their products in the US. Why bother investing in making something fancy when the only legal place to get their products is from them. There's no competition for their titles, and most all of their customers know exactly which product they came to the site to find out about.
Aside from what was noted about them also being available at Amazon, I also did a quick check at RACS (Roberts Anime Corner Store) and found Aniplex titles there.
I'm less familiar with Pony Canyon USA's releases, but the one I knew of also turned up as available through RACS. I also found that same title (Denki-Gai) on Amazon.