Is it Ethical To Use Cross Domain Rel=Canonical Tags On Websites?

[baldwinjackson]

Hello folks I have been researching for a while against industry giants link building strategy and found an strange practice which initially looks grey hat.

For suppose you are website development agency which build new website for different brand. Your business website is:
example.com

Every time you build new website for stagging purpose you create sub domain on your website then push it live on main domain.

clientwebsite.example.com

Note the above website is sub domain of example.com

After making it live by default you put rel="clientwebsite.example.com" tag in every client main website.

What I have observed is that rel=canonical tell Google that there are multiple copies of same page referring the actual version.

Similarly when any client website build links it pass the authority to sub domain of clientwebsite.example.com and links can be seen in example.com profile by using any backlink tool.

Strange but fishy technique to build links. Whereas when you search the client name in Google you get actual website.

Note: Both the version are getting index by Google I have manually check around 20 to 30 different sites.