Being pro-FOSS does not mean you are anti proprietary software. There’s plenty of middle ground.
Video games, for example, where the company might not want to open source their server code for plenty of very legitimate reasons.
Mail is a super important utility. For physical mail, it’s a felony for anybody to open it. For digital mail, gmail open, reads, and inserts ads before handing it to you. It’s ok to think some things are important enough to mandate FOSS, and some things not.