at least for Mozilla/Gecko this interactivity problem within the img element
might be related to security problems.
In the past (since about version ~9, before it worked, no progress up to
today) interactivity was switched off in Mozilla/Gecko in general and
intentionally due to security problems related to (X)HTML forms and event
Interactivity was surprisingly connected to the question, whether users allow
the interpretation of scripting or not - if they allow script interpretation,
interactivity is available in standalone SVGs and embedded SVGs
(obviously no security problem) but is not available, if script interpretation
Presumably, because there are concerns about script interpretation in
content referenced with the img element, there are similar security
problems for SVGs referenced with the img element, script interpretation
is switched off for such content, therefore interactivity does not work.
However, other user-agents seem not to connect interactivity to scripting,
for them interactivity works in standalone SVGs or embedded SVGs, they behave
as specified and expected by the recommendation. Maybe they
have another reason to ignore it for content referenced with the img element.
Some extensions/applications for user-agents seem to have problems with
the object element as well, there seem to the fewest problems, if one adds
directly the SVG content to an XHTML document as a fragment without using
specific XHTML elements, however this can be inefficient, if one wants to use the
SVG more than once in a project.