At £20 pa, with £10 concessions and £5 juniors, it amounts to a nominal charge that brings in some revenue from canal stretches that are not leased by clubs.
If there are no such canal stretches nearby, it obviously wouldn't be good value. But it may be a very useful arrangement for people in other areas.
A browse on the CRT website will reveal a surprising amount done, on a number of fronts, to further the maintenance of canals and their promotion to the public. To take one instance, buried in the details of their website and possibly not widely known, a friend who's an angling coach regularly takes part in Learn to Fish taster sessions, where each coach will spend say half an hour with children new to fishing, and try to ensure they catch a fish. A lot of care and planning goes into these events, there are hundreds each year around the country, and they don't organise themselves.
With a possible alternative of a free for all neglect of out canals, I don't think the small charge for fishing stretches where they don't get revenue from club rents is unreasonable.