http://www.broadcastingcable.com/news/currency/twitter-makes-deal-stream-live-mls-soccer-matches/172283
According to the article that the MLS has a deal with Twitter for broadcast rights. Previously Facebook held the broadcast rights to MLS Games.
Should note that this is only for the 25 matches broadcast nationally on Univision (in Spanish with SAP English audio available). Local matches are still on local providers (including YouTube TV for LAFC and the Sounders), and national carriers like ESPN and FS1. Starting in April, ESPN will also stream local matches on ESPN+, a replacement for the MLS Live streaming service.