There is a subtle difference between the existence of an architect in the history of a house and an SEO in the history of a website.
There is no reason to avoid signalling that a house has been designed by an architect in the same way that there is no reason to avoid signalling that a website has been professionally designed or coded or that the copy was written by a professional writer.
Signalling SEO involvement is similar to signalling the involvement of a soliciter in an apology statement, an advertiser in an article or a 'campaign donor' in a politician's policy. Every one of these parties is likely to claim that they are (in SEO terminology) white hat, they follow ethical standard and they do not comprimise the integrity of the article, policy or apology. They are deeply concerned with the effect of these on the recipients (readers, citizens, victims or users in SEO terminology). However, the recipients cannot be blamed for their skeptisism, both because of anecdotal evidence & by an examination of the interests of all parties involved.