With TapBots having made very successful Apps in the past, I'm pretty sure they'd have the cash!
You're wrong. It costs a lot of money to host a server and we don't know how many people actual use TweetBot. For them to pay for a server and then only 2000 people use their client.. It's not worth it. They would be losing money. Tapbots is run by two individuals that do all the work themselves. They aren't a big company.