Having compatibility pack may actually help TN to make more money off DLC (in the US market at least) if the other player gets to see the costume in action. Still images don't really tell much about the costume, and that might turn some people off.
However, the fact of having the costume data available means that the user can simply extract the costume data and swap it onto existing costume slot. TN is basically trying to prevent that from happening I guess. I don't really want to go into details but this is what I've seen with other games.
As for showing off, I bet most Japanese gamers are more into
ryona anyway so they don't care about online play lol, or they're so competitive in such way that they don't care about costume at all. I am mentioning "Japanese" because Japanese customers are less reluctant to buy DLCs due to higher economic standard: everything there are priced so much higher that 500 yen doesn't mean much to them.