I agree with this - and not just coccidiosis but e.cuniculi as well. They are both really nasty infectious diseases which can easily be transmitted across grass and both diseases are common, and common in rabbits who are not displaying symptoms of illness. Even if cocci is more prevalent in wild rabbits, it's the fact that it's a confined area with intensive use that raises the risk, and e.cuniculi spores are spread from urine and more prevalent in domestic rabbits, which means that it's extremely likely to be passed around by rabbits weeing on grass. Because they can both be passed on by rabbits who are not ill, and because illness does not come on suddenly and quickly (it could be years for ec) it would be entirely possible for a boarder to operate this way for years without realising that they are actually the source of transmitting infectious diseases to clients rabbits.
Having seen first hand how absolutely devastating coccidiosis can be (and e.cuniculi for that matter, but not first hand fortunately) there is absolutely no way I would allow my bunnies to go out onto grass runs at a boarder. The intensity of use of runs and the high turnover of rabbits in those runs raises the risk way beyond that of a normal patch of grass. Having said that, the local boarder that I have used occasionally does have grass runs, but mine don't use the grass runs and instead use the part of the accommodation that is large enough for them to stay all the time, and she knows and understands why I don't want them on grass and respects that. It's not ideal because what I really want is for a boarder to understand that risk for themselves and to choose not to do it at all, not just because a customer asks, which is why I travel a long distance for boarding for any longer period of time.
My personal preference is for bunnies to board in accommodation that is large enough for them all the time without the need to stress them out by moving them between accommodation and a run. I think this keeps them safer from infectious diseases, minimises stress and also minimises the risk of them getting upset by the sight of other rabbits in the area. I just don't think it's worth the risk, especially considering it's not difficult to put those runs onto a hard area instead.
The other thing I'd add to things other people have said is to see what other pets they have/boarders they accept. A lot of boarders seem to have dogs, which is fine as long as the rabbits are fine with that and/or the boarder keeps the two well separated. I also wouldn't board anywhere that accepted ferrets in boarding, although fortunately most bunny boarders do tend to stick just to bunnies so it shouldn't be too much of a problem - something to bear in mind though.
Good luck with finding somewhere you're happy with