Research encounter, level 15 pokemon

To eat up your $tardust

Level 15 plus 100% BCR makes 99.96% capture rate without any throw bonuses.

Most likely through balancing purposes-

Think about it, it's a shot to get a good IV of such species (same as raid/egg IVs if I'm not mistaken?). They are often not competitive, but it's essentially doing a free raid or a free egg, provided you catch it. I think the 5 level setback is not too hard to swallow- the stardust cost is trivial anyhow.

I think there's a quest in the questline that requires level 15. Perhaps, this time, they don't want a level 15 person having a level 20 Pokemon until they level up (Though they are fine with a level 7 account critical-catching a 2877CP Weather-Boosted Groudon and being set for life)

Quest encounters are more abundant, easier, and cheaper than raid encounters or eggs.