Given that no-one has attempted to justify such actions, can I suggest...
Yeronner, may I suggest that you consider the exchanges in posts #42, #54.
Further, I submit that I'm not arguing that posters here have justified it, but that they offer impedence of (motor) traffic as a reason that an irritated motorist may use in justification.
If a journey was needlessly delayed by someone who was travelling unnecessarily slowly because they were concentrating on something completely unrelated to their journey, would they get frustrated?
Note that there is no mention of the respective forms of transport, what else they were concentrating on, what speed they were actually doing and no suggestion of anything beyond frustration.
Someone who sets off, in a car, on a journey to pretty much any urban centre in the UK these days is kidding themselves if they expect not to be delayed (travelling in the wee small hours of the morning excepted).
I have to agree that at first blush, the answer is "yes". But look at it again - "unnecessarily slowly" is a bit of a moveable feast, isn't it? On a laden tourer, I'd possibly be doing 10-12mph. The lady I see on her shopper in Wilmslow won't manage much more than that. I daresay the roadie on his mobile wouldn't have been doing less than either of us. As another poster pointed out, somehow an unfit/non racing/laden cyclist doing that speed is ok, but the chap on his mobile not[1].
My objection to the original post is twofold;
1) The cyclist in question (taking the op's post at face value) poses far less of a risk to others than any number of disturbingly prevalent behaviours exhibited daily by drivers. The idea that the former should be thought to give "us" a bad name is one that should be resisted.
2) That the impedence of (motor) traffic is irresponsible and discourteous, as though the cyclist has no right to make progress on the road.
[1]I do find the use of a mobile phone by any road user whilst they drive/ride problematic, although I'm less concerned by a cyclist (has to slow down, is relatively easy to avoid) who does it than a motorist (using the mobile has little effect on speed, travels faster, is heavier and harder to avoid), personally.