For alias addresses they are filtering that because the data thiefs and middlemen know a small portion of people do this. They cut off the plus or dot part when your domain has a well known MX/MTA connected to it.

That still leaves custom mailservers and non-alias (as in aliases via plus or dot, not actual mailserver aliases) which does give an indication as to who is sharing your data.

A decade ago I found the unique email address I used at Hertz.com leaked for spam.

Of course when I complained they said it must have been something I did...