The gate opens the archive instead of reading the file extension
Why
92 of the 93 live Blender products ship as a .zip. A gate keyed on the extension - the way the Unity one is - would have fired on almost none of them. Opening the archive is also what makes the headline check correct rather than merely strict: a texture that is not packed into the .blend is not a defect when it is sitting right beside it in the same download.
What it cost
A zip reader, on bytes a stranger uploaded, with every bound that implies. It claims an archive only when a .blend or an add-on manifest is genuinely inside, so the hundreds of ordinary zips in the catalog are read once and then left alone.