There are limited circumstances (depending on how the company is set up to accept credit cards) where this could be tagged as something other than a normal purchase, and then it's up to your issuing bank to decide how to handle the charges.
Frankly I don't know why you'd invest using a credit card. These are startup companies in most cases, many of them a LONG way away from being anything remotely close to a publicly-traded stock where you could sell your holdings, so you can't rationalize using a credit card by the return you'll get (unless you're going to pay the bill in its entirety, which still invalidates the point of using a credit card in the first place!).
If I had to speculate (and this is speculation!), I'd say the reason they let people invest by credit card is to make it easier, for starters. It lets people make a quick, simple transaction when this should be a carefully thought through decision, because you're essentially throwing your money away with many of these companies simply for the vanity of being able to say you invested in them.
After glancing through the site, I would advise you to be careful about investing, mainly because there are heavy restrictions on what you can do with your shares for 12 months after purchase. Additionally, there is no liquid market for your shares in many (if not most) of these companies.
As they warn, don't invest any money that would require you "alter your lifestyle", which is another way of saying don't put money in that you can't afford to lose or walk away from.
If you can afford to speculate and put a little bit in a few of these companies, however, it could work out alright, but the odds are long. The majority of them are pre-revenue.