"is head of xyz since" vs "has been [...] since" ? Is "is" ever acceptable?

Over on the ELL StackExchange, this broad type of construction has been declared unacceptable (see here). The same claim is made in this blog by a native speaker who is an editor and language trainer; see here.

These verdicts notwithstanding, the construction does seem to be at least marginally acceptable, at least in American English.

Discussion

In the Corpus of Contemporary American English (COCA), we find this attested example:

Since 2004 he is a partner and CTO of Secure Network, a firm specializing in…

As indicated in the question, one can find examples of this construction on web pages of organizations where there is little doubt the writer is a native English speaker. One example:

Sharlene S. Vichness is Founder and President of Language Directions, LLC. since 2005. (source)

Here is a relevant section of the CGEL (p. 697):

Since, irrespective of the type of complement, is largely restricted in BrE to occurrence with the perfect, as in [13i] [Jill has sold over 200 policies since she joined the company.]; it can, however, be used with simple tenses in the construction It is now nearly a year since he died. AmE allows preterites rather more widely: ^%Since you went home we redecorated our bedroom.

(Here ^% means 'grammatical in some dialect(s) only'.)

Also, the Macmillan Dictionary has this note in its entry for since (here):

When since is used for talking about time, the verb in the main clause of the sentence is usually in the present perfect or past perfect tense: It hasn’t rained since the end of July. He had been composing music since he was ten years old. Sometimes the present simple or past simple is used in the main clause [emphasis mine]: It’s over twenty years since we last met.

I should stress that this doesn't mean that there is a free-for-all on the usage of present simple with since. The marginal acceptability of the examples above seems to be constrained to the particular narrow context of biographical entries. For example, I don't think any native speaker would find the following acceptable: *Police Chief Smith fights crime since 1990. It would have to be has fought or has been fighting.

Having said all that, the corresponding construction with the present perfect (e.g. Since 2000 he has been a director at Acquavella Galleries, New York, which…) is much more common in COCA.

Also, in the American Heritage Dictionary entry for since, all the relevant examples use the present perfect; see here.

In conclusion, lots of people will agree with you that it is not acceptable. Nevertheless, it is not unheard of for native speakers to use this construction. So, I conclude that it is marginally acceptable.

(Note that related questions pop up in various forums from time to time, e.g. here, here, here…)