Open source code can be legally copied and re-packaged by anyone. It’s kind of a moral question if due credit should be given to original authors. A huge majority of Wp.org plugins have copied code from non-open source codes. Even after identifying and complaining on wp.org, the plugins still get approved. However, another way to look at this, is to be proud that big companies value your product which you can show as your strength. Also keep faith in open source that if you can keep your motivation and grow your community, you’ll have a better product.