A lot of commercial sites do it because of lost ad revenue. Some of them, like Ars Technica, give you full-text RSS feeds, if you subscribe to them. I think that's fair.

There are some non-commercial blogs with no ads, nothing but text on them, but still giving you only the title in the RSS feeds. I suspect it's the default configuration of whatever software they're using for the blogs, and they may not even be aware that RSS is a thing. Maybe I should start writing mails to them to inquire why that is and maybe change their minds on that, since I really can't see a reason for it.

FWIW Most of the feeds I subscribe to are full-text.