Most of the Crowdsale websites have a signup/login page where the user has a wallet and several options of purchasing tokens. Options usually include Bitcoin, Ether and Wire Transfer. No MetaMask or other client is required to interact with the crowdsale website.
My educated guess is that this is a Dapp, not much different that Truffle's MetaCoin example, with an Ethereum Client Docker container hosted on a server to interact with the Crowdsale contract through Web3.js in the server-side. I also guess that they use BTC-Relay to accept Bitcoin. However, I can't wrap my mind around the details.
Please include technical details if you can. Thank you in advance!