Frowned upon? Yes.
Illegal? No.
The dividing line is if you use any of their support or services. Cookies, donuts, refreshments, mechanical support, etc...Is a direct function of the entrance fee.
What is more abstract is the route planning, their permits, security, traffic direction services like barricades & policemen. This is a function of being part of a human herd. On the one hand, you benefit from safety in numbers, they benefit from more cyclists being visible from a public relations/visibility/influence standpoint. The cost is your presence is unaccounted for so accounting cost/benefit analysis on the backend is skewed.
To avoid "theft" of the organizers work, I say don't be at the start/finish line. Don't use any services. Ride your own route that is at best coincidental (at most 50%) to the ride taking place. As if you were independently out for a ride & just happened to come across the group. Which legitimately does happen from time to time. Otherwise, just leave well enough alone.
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I shouldn't have to "make myself more visible;" Drivers should just stop running people over.
Car dependency is a tax.